Shields are terribly janky for a number of reasons. I can see them being decent backup options for non-weapon-using bards, clerics, druids, and Fighter Dedication sorcerers and wizards. Otherwise, they are fairly bad for fighters and paladins. They are good for one thing, and one thing only: helping prevent an alpha strike from a high-initiative enemy during the first round of combat, presuming you have an action to spare before said first round of combat.
First of all, so long as you are holding the shield, you take a -1 armor check penalty, and shields are not actually armor, so expert shields do not eliminate this penalty. Shields require an action to raise, and that is rather burdensome when actions are golden during combat. Shield AC bonuses are circumstance bonuses that do not stack with any others, such as cover or screening.
Furthermore, Shield Block requires a reaction (which means no Attack of Opportunity or Retributive Strike), and it prevents you from taking damage only up to the shield's Hardness in damage. The shield takes the full damage, or at least, that is what page 175 explains, with the example of a Hardness 3 shield taking 10 damage. Once the shield inevitably becomes broken, as per page 175, it takes an action to unstrap it, and that is presuming you have a free hand with which to unstrap it. You also need to spend money to keep your shield up-to-date, lest it fall out of usefulness, and you need to constantly repair that expensive shield.
Let us take all of the above into account. A 5th-level fighter or a paladin spends a hefty sum of 900 sp for an expert light shield steel sturdy (level 4 item), with Hardness 8. Yes, it comes only in light shield form. It imposes a -1 armor check penalty. They need to spend an action to raise the shield for an amazing +1 circumstance bonus to AC, which does not stack with any other circumstance bonuses, such as cover or screening. If they blow their reaction (no Attack of Opportunity or Retributive Strike), they can perform a Shield Block. Whenever they Shield Block, they prevent a measly 8 damage (not much of an improvement compared to the 5 damage prevented at 1st level); the shield takes a Dent if the attack dealt 8-15 damage, or two Dents for 16+ damage, and the magic shield is broken at three Dents. Once it is broken, it is dead weight until unstrapped with an action, and then later repaired.
A fighter might even think themselves clever by trying out a Double Slice build with a light shield boss/spikes. That may work at 1st or 2nd level, but by 3rd level, Weapon Expert means that sword pairings, hammer pairings, or axe pairings become more accurate and optimal. Indeed, when I was running my second group through Doomsday Dawn: Part #1: The Lost Star, that is exactly what the 1st-level fighter tried. Suffice it to say, it was rather anticlimactic when their hardness 5 steel shield was broken by a single 10 damage hit in the very first round of the first combat, since it was not even a magic sturdy shield. They had to resort to a golf bag of backup shields to strap on in different fights, and that tactic becomes unfeasible later, when only magic sturdy shields are relevant.
This is awful.
