I'm a bit into act 2 now, so that is as far as my experience goes but i have noticed a trend of issues surfacing while playing my mage character. Especially in contrast to my party (playing 4-player coop, the hardest part is organizing a time when we can all play).

To start off it would appear that spells scale very poorly. Physical build scale well with weapon, with their stats, with skill schools (especially Warfare for double dipping it seems), and Critical chance. By comparison my spells only scale with Inteligence and the Skill points, and no such generous scaling as with Warfare. My spells cannot even natively crit!
In top of that every single enemy encounter we run into the enemy seems to have a resistance to atleast one of my schools, often several and fairly high. We're just playing Classic mode here, why are there so many enemies with 40%+ resistances? Seeing physical resistance is exceptionally rare, the one time i did the enemy also had even higher magical resistances.
My spells already scale poorly, why are they then punished extra hard with constant heavy resistances they have to try and punch through?

The next issues is the cooldowns. My god are the cooldowns long. Fireball, a fairly basicly, early-game, straight up damaging fire skill has a 4 turn cooldown. It doesn't even compare to a regular attack from either the Ranger or the Rogue in our party, and even the Tank deals more damage with her physical abilities.
Pyro has only a single damaging skill with "just" a 2 turn cooldown and that is Ignition. The other spell schools don't have any at all with less than 3 turn cooldowns. Especially daunting when enemies have such high resistance to one or more elements that those skills might as well not even be on my bar. Which happens quite often i have to say, in particular Fire seems very commonly resisted (to the point of healing enemies even).
To compound this the AP cost of spells also seems way to high. Without any extra AP generation from Haste i can sometimes do little more than throw out one spell in a turn. Which then gets put to shame by our Ranger or Rogue, who happily keep throwing out that kind of damage every turn.

Our Ranger can throw out Reactive Shot. Let's compare to Fireball. Both req. 2 in their respective skill trees. Both inflict AoE damage upon cast, both cost 2AP, both are Ranged. However Reactive Shot does more damage, has only a 2 turn cooldown, targets enemies for more damage if they move (vs. Fire field), as a Physical skill doesn't have to worry about resistances, and is smart enough not to hit allies.
Basicly a much better version of Fireball outside of very specific situation where enemies are desperately weak to Fire or you need a Fire field for some reason.

The fact many spells can be a real liability, or have mechanics complicating them further also doesn't help. For instance Laser Ray, nice spell but its outright unusable when i am on an elevated terrain. Which is where a ranged caster is suppose to be, right? Nether Swap, nice utility, if it works. Which it often does not. Fireball, mediocre damage skill with some AoE fire field. Goodluck not hitting more of your allies than your enemies. Rain+Electric combo, water spreads where it wants to and you just don't know ahead of time if you end up accidentally with more of your allies standing in water.
This doesn't exactly help trying to strategically plan your actions ahead of time.

I think Warfare needs to be fixed, physical damage is far to powerful as it is. Savage Sortilege should be a baseline Talent, spells should crit natively. Maxing respective skill trees (i.e. 10 Pyro) should reduce the AP cost of the skills within that tree. Cooldowns need to be reduced for many of the damaging spells. Something needs to give penetration for resistances, perhaps as a stat always present on Staffs. A weapon that currently feels very underwhelming for Spellcasters. This helps Staffs be more usefull, and helps Spellcasters with their problem of benefiting very little from their weapons. Point in case, im still using a lv8 staff because it has 20% crit chance while i am actually level 13. Simply haven't found anything better yet as Weapon damage is inconsequential to me.