Demonologists are great if you get them early. Their demons are expendable and they will take great numbers with them in death. But late game they probably won't be that great. If you have real demon stacks, you would have to be careful with them, since ressing them is hard as already mentioned. I like getting no losses because of the LS bonus and that is needed in harder difficulties. On normal you can probably beat the game with a couple of thorns and no trap bonus at all.
There are quite few enemies with magic immunity/resistance in the game. Only dragons, mages and gryphons and you don't fight those very often. There should be a unit vulnerable to magic in any army, if you don't attack a dragon's den.
Runemages are not that great imo. Their revive is weaker than inquisitors and their beam is also quite weak unless you have a lot of unused runes. I would prefer to use all my runes however. Their phantom skill is quite useful though.