Whew, a lot of good feedback. I'll try to explain some stuff and give my reasoning to some of my decisions with the caster characters, but I'm open for further discussion. :)
Let's start with the warlock. Warlock is a bit more complicated in terms of balance. Some time ago he had +2 damage on firebolt, and he just blasted through everything early before things could get near him. The thing is, that yes he requires more stamina than the rest of the characters, because he doesn't do well in regular attacks, but he gains range and no ammo requirements in return. In open-world range is a huge advantage over what melee fighters have, and archers require ammunition. Abilities also have a 100% chance to hit, and regular attacks don't, that also adds up to the cost. So stamina is the only resource a Warlock has to worry about for his power and since the cost of abilities doesn't go down with endurance like regular attacks.
One interesting thing you might not have come across yet are magic weapons. They are not available as often as standard weapons, but many magic effects add additional damage type or change it completely (for example, a weapon does only Holy damage instead of Physical). These weapons scale with Intelligence instead of Strength, so if you need to save up on stamina, this might be a choice when things get too close. Magic items can be crafted too, but it's luck-based. Some basic weapons like ceremonial daggers have extra holy damage without any additional affixes rolled.
Protections are not as strong as you think. Most defense comes from willpower because it covers the whole body. Shirt will only add it's protection when you hit the enemy in the torso. You can see which slots protect which parts in your inventory screen under "resistances", hover your cursor over some values there and see the magic. ;)
Necro is even worse to balance. I'm a big fan of WFRP, and for a long time, I've been fascinated by the story of Nagash. I want to have a necro that can create his horde of the dead, but, because this is a game, it would be too easy if these zombies could be powerful enough to do all the work for you. So the idea here is that you can summon as many as you can get corpses, but they are disposable. They will die quickly, they won't do much damage, they will soak some damage, but they also are almost free to make, because not all corpses can be butchered or turned into other resources. There are also some options you could try, but maybe not reliable. There is an AoE healing ability in the game that heals you and zombies, so with right combinations, this army might spiral out of control. :)