Like with the fighter and reactive strike, there is the problem of pricing in the prerequisite of enemies allowing you to trigger this.
Part of the champion's reaction is the idea that you make enemies choose between suffering for attacking your allies (while doing less damage to them) or attacking you, who will be well armed and resilient.
Even halfway intelligent enemies will likely pick the latter after one turn of suffering from strong reprisals.
There is also the issues that depending on what attacks you use it against, your shield could quickly give out, since at least one of your reactions has to include the shield block.
With the recent errata, there is also the benefit of blessed armament doing more for you, which you will be trading in for the blessed shield, including its class feat chain.
You do require three champion class feats and one archetype for this, which you then need to compare to the most damage optimizing feat choices and archetypes you could add instead.
Also, to avoid some penalty, you would need something like the agile shield grip feat to fully make use of double slice.
Barring that, you would effectively trade a +3 to hit (-2 attack vs -5) for the lower damage of the shield boss / spike on top of the issue of likely lacking property runes for extra damage on the shield until higher levels when the better doubling rings come into play.
That can make the exact numbers at least a bit tricky, especially if the potential factor of damage resistance comes into play making many weak attacks less advantageous. And unlike other abilities, DR is relatively common to face.
While I would not dismiss the point, it at least is not fully as straightforward.
You would really need to fully stat out the optimized 1handed weapon and shield vs 2handed weapon build, including obvious choices for archetype to maximize damage for a fair comparison.
Selfless (champion) - a champion who defends their allies with their reaction by mitigating damage
I usually use the term altruist for these champions but selfless is not wrong.
For the rest of the post, I will be focusing on justice champions, unless specified otherwise. Why? Because they are the ones suffering in a bit of identity crisis. They are defenders, like the other Selfless Causes, but their reaction and reputation in the community leads them to be thought of a damage dealers or Threat-Tanks. I identified two primary issues with them, and surprisingly both relate to their weapons.
I don't know if call it as a "identity crisis" because the role of champion having to be a tanker inspired in MMORPG class comes from the community due the fact that champions is one of most defensively stronger class that punishes enemies who tries to attack other PCs while protecting them using their reaction what stimulates intelligent enemies (and GM) to focus their damage output on them instead of other PCs but nothing of that says that champions cannot do some good damage like an old Lineage 2 tanky class being perfectly fine that champions can have some good offensive power. Including I have a impression that Justice Champions (also called Paladins in legacy) are the most played champion subclass due this.
Part 1: The One-Handed Problem
Justice Champions, if looked at as damage dealing threat tanks often gravitate towards bigger, two handed weapons, especially with the reach trait (see part 2). This however, in the current state of the game is misleading. Because of the Shield of Reckoning feat-tree (Blessed Shield -> Shield Warden -> Quick Shield Block -> Shield of Reckoning), justice champions with a one handed weapon and shield can start using an additional reaction per turn, starting at level 10, compared to all other champions, who only get an extra use of their main class feature at level 14, taking the Divine Reflexes feat. The extra reaction is absurdly powerful, giving them an extra attack unaffected by the Multiple Attack Penalty. This progression is only matched by the Fighter, the most famous and effective damage dealer in the system. Champions however, can only do this with a small weapon, unlike Fighters, who can run around with a d10/d12 weapon Reactive Striking left and right. If this doesn't invalidate two-handed justice champions enough, a shield attachment and a single class feat sacrificed gets the one-handed build Double Slice through the Two Weapon Warrior Dedication at level 2. With a rather simple setup like this, our one handed champion gets to make 4 MAP-less attacks at level 10. Yes, 4. That's more than most people make normal attacks. Meanwhile, our two-handed champion is stuck with a free rune on their weapon, no special actions and no way to match the scaling of it's one-handed sibling.
I see this as a possible balance concern and problem because most people believe (myself included, I only found out about this interaction a bit ago) that a two handed champion should deal more damage. Sadly, most of these people end up with fairly underwhelming characters that have sacrificed their defenses for nothing. Yes, that's the issue. A one handed champion gets to keep their defensive upside while also dealing more damage. The sacrifice of using a two-handed weapon somehow gets it behind, not ahead.
Yes yet I don't know if the use of 2-handed weapon is really an alternative focus to the class but just something that is possible to do. IMO it's just like fighting with a dagger. It will work but probably the finesse and agile traits won't worth for justice champions they lower damage for a character that usually have a low amount Dex and/or does a low number of attacks with MAP to really worth.
Champions is a class very focused in using shields also to improve their survivability due their reactions calls too much enemy attention. IMO if the idea is to make a two-handed champion more damage focused probably a champion MC archetype is a better idea. This would allow the character to have a better offensive power with such weapons while still punishes enemies who tries to attack allies. This a very strong build for both fighters and barbarians.
Part 2: The Reach Problem
Now, let's start talking about the biggest issue the justice cause has: reach. A High-OP justice champion simply can't be effective without a reach weapon, no matter the hands. This is thanks to their unique reaction, that not only requires the triggering enemy and hurt ally to be in your champion's aura, but also for the enemy to be in your weapon's reach. People are quick to jump to the Nimble Reprisal feat as a solution, but it's sadly only making the problem worse. While it tries to help non-reach weapons compete with reach, it makes reach weapons overwhelmingly stronger. As an example, if you're a justice champion and you're standing on a diagonal line, with an ally in the middle and an enemy on it's other side, even with nimble reprisal, you cannot step anywhere in a way your enemy ends up in your reach. Reach weapons however, don't struggle from this. On top of that, reach weapons let you cover and threaten more od the battlefield at any time, making you just better at your job. Workarounds of throwing weapons and/or getting Enlarged all have fairly major downsides, such as making said attack with Dexterity or being Clumsy for the duration.
You still have the option to use a Gnome Flickmace. It's not hard to take once it is an advanced weapon with ancestry trait so you can easily take it playing as a gnome or using Adopted Ancestry and taking the Gnome Weapon Familiarity. This was a so good alternative to give champions more reach that IMO Paizo designers have nerfed it from d8 to d6 + sweep exactly to diminish the must have sensation for champions when they made this change in CRB 4th print once that are champion build was most prejudiced by this change because shielded champions rarely uses 2 strikes in their turn. Yet just lose 1 avg damage per weapon damage dice (d8 to d6) still worth once that the alternative of use thrown weapons (trident) is a MAD option that makes you sacrifice other things (usually your max HP due lowest Con) and also is not a way better damage (due Dex becomes your key attribute instead of Str you have -1 damage during many levels and -2 in the end of the game so invest in a thrown weapon for champions it isn't a so great benefit).
justice champion are far better at damage than any other subclass
I no more agree after remaster due Expand Aura. Have a 30 ft aura is very very good thing to have with a champion but for Paladins Justice Champions try effectively fully benefit from their reactions to protect an ally far from 20ft means that you need a thrown weapon and even the strongest thrown weapon that is the Trident will have a lowest hit rate if the enemy is far from your 20ft weapon range. Yet Expand Aura is so good that it still worth for Paladins Justice Champions but I also start to consider that other reaction options like Flash of Grandeur and Glimpse of Redemption are better alternatives for such range.
all champion can also get the new draconic barrage
Not all only those who choose to worship a deity with a dragon domain.
I play a Dual Dagger Justice Champion in Age of Ashes alongside a Shield of Reckoning Justice Champion.
In terms of damage, the Shield of Reckoning Justice Champion is ridiculous. He makes one attack per round in general (having to Raise their Shield to get their juicy reactions), considering that his reactions are not triggered that often (especially Shield of Reckoning, adjacency is hard to get at level 10).
As I can throw my daggers I trigger more free attacks from Ranged Reprisal and still don't get it every round, far from it.
A Shield of Reckoning Champion is in the extreme low end in terms of damage. Also, reactions are very far from a given, the only ones that can be triggered consistently are those akin to Opportune Backstab or Topple Foe.
As a side note: Despite its rarity, Shield of Reckoning is massive when it triggers. And the Shield of Reckoning Justice Champion is an absolute tank, there's no doubt about that. But it's nowhere close to an average damage dealer.