The biggest issue to me is that they are WAY too easy and their skill tree lacks flavor. In fact, I only got the pack after I made sure there are mods that introduce more challenge. I have never even experienced the vanilla gameplay and I still felt like it was too easy. Though even disregarding difficulty, this occult state was still disappointing to me because they just lack flavor. There was no roleplay/morality choices like there are in Werewolves and especially Vampires. In Vampires I love the fact that you need to pick weaknesses that can then cancel certain strengths. There's also a variety of Aspirations to further improve your roleplay.
With Spellcasters you can't pick and choose traits most appropriate for your character since the trees force you to advance in a certain order. Also, the skill tree just lacks imagination and is very basic like "you earn Xp faster" or "your spells are less likely to fail" or "you generate less Spell charge" *YAWN*. Compare this to Vampires and Werewolves who actually get new abilities and interactions.
Learning of spells/potions is also way too simplified. Spells could be a great customization option but instead you learn them at random and thus basically every single spellcaster knows the exact same spells. The only difference is whether you choose to ignore certain magic schools. I guess you could bypass this by only learning spells via tomes but my point still stands. As it is there's not even really an incentive to use the freaking tomes since you learn everything easily with just practicing spells by yourself. Boring. Also, what is even the point of becoming an alchemist since you get free samples of powerful potions and can just then use Copypasto spell again and again? (One of the things a mod eliminated.)
I still managed to have fun with my spellcasters but it just kinda took a lot of self-imposed restrictions, roleplay and mods.
