Playing on hard for the first 5 levels was absolutely amazing! I got massacred so many times, it was fantastic! On average I think I had to try each encounter ~2-3 times before I could win, and even then it was with potions and the occasional resurrect scroll. I was having the most fun I've had with a game since Baldur's Gate. But as I've continued to level, and more importantly get gear with resists, and the witch spell that buffs resists, I am now butchering everyone on the first attempt.
I've heard some people say they find the game too hard, but I've also seen a lot of posts similar to mine where the complaint is that after the beginning of the game it becomes easy mode.
I could try to use a less optimum blend of characters and/or abilities/gear, but the joy of a tactical game like this is often in finding a great strategy.
I would recommend either adding to the current hard difficulty, or making another difficulty after that ("insane" or whatever) that has the penalty that all resists are halved. So at the beginning when you have 0 resist, you still have that, but getting yourself to 100-120% resists would become much more challenging, and more importantly, would force you to choose only one or at most two resists to get that high, rather than being able to just become a massive resist beast.
Absolutely great game though!