I really like the Arcanist as it has a lot of possible customizations through Arcane Exploits.
The largest weakness of the class is its dependence on two major ability scores, Intelligence for spells and Charisma for class abilities. Charisma is the least beneficial mental ability score, and spreading out abilities scores is both expsneive and put you at risk of not being effective in either.
A major drawback is any of the Arcanist archetypes is they trade off the Arcane Exploits for hit-and-miss abilities. Those along with many existing Arcane Exploits are usually dependent on expending Arcane points from the classe's Arcane Reservoir.
Limited Arcane points is another major weakness as unless you cannibalize your own spells for more points you can quickly run out. Even if you do there is a limit on how many times you can do it per day. Plenty of abilities that can become useless.
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There are several very useful Archetypes.
Brown-fur Transmuter specializes in using and sharing otherwise self-only buff spells.
Occultist can trade Arcane points for an equal level Summon monster.
School Savant gives extra versatility with one more spell prepare per day, but also can give some very useful magic school abilities.
Twilight Sage is the only archetype that actually gives a steady source of gaining more Arcane points to use abilities. It comes at a heavy price. After Consume Spell became limited per day it changes the balance between the archetypes.
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The Twilight Sage is not the most powerful but all the other archetypes are heavily dependent on limited Arcane points. Is it must-have to power all the Arcane exploits?
The Archetypes can be very useful. Trading exploits for archetypes is a drawback, but some are worth it.
The problem is how to change the Arcane point economy. Consume Spell and Consume Magic Item arent efficient methods to gain points and using them requires Charisma, money, and a generally complicated system.
The Twilight Sage is the only archetype that works to add more Arcane points rather than more abilities to spend them on. Other Archetypes have very useful abilities like the Occultist essentially having Summon Monster 1-9 as bonus spells. Only being able to use it about once per day is a waste.
Are there other reliable methods so the other Archetypes can use their added abilities fully?
There is one more method of getting a ton of arcane points: fiendish proboscis. It allows you what amounts to unlimited out-of combat Arcane pool points, and is kind of broken, but still really cool.
Spell specialist into dragon disciple can bootstrap itself into the draconic sorcerer bloodline, with the bloodline development exploit. You can also stack spell specialist with brownfur, but that will leave you pretty exploit starved.