HOMM4 had many new great features to offer. However instant enemy retaliation wasn't that great - you were much more certain to suffer own unit losses than in the previous games. And the Resurrection spell for heroes was removed too, Angel units were the only ones left with ability to resurrect friendly units. Which was altogether a huge mistake in my opinion.

Ever used the spell Divine Intervention? Perhaps only available for Life, it resurrects all the heroes and gives a large amount of HP to them. But it's still there. The problem is, that the fanatics couldn't bear the changes. I'm mostly a fan of HOMM4 because it was the first of the series I played with, but I was at least not blinded by prejudicement.

Oh don't worry, I like Heroes of Might & Magic IV not any less than HOMM3 or HOMM2, there are simply many new things you can do in it which you couldn't in previous games.
Considering that with a high level Demonologist hero who masters Demonology and Nature Magic (summon Poison Spitters, Ice Demons or Devils) or with an Arch Mage hero (3 or more magic schools, +20% effect on all spells I think) with Life, Order and Chaos magic (you can add Nature to that, but not necessary) you can pretty much solo almost any battle, if you don't also forget to master his Combat skills and maybe Tactics, this alone is a pretty different experience from previous games in the series, and a huge freedom I very much enjoyed to exhaust to it's limits.

Well considering that it seems that if HOMM4 succeeded, then HOMM5 could have been a TBS/RPG hybrid. Which could be awesome! :D

I just remembered a post on GOG (or somewhere else) regarding how HOMM4 is undervalued. From the time perspective (also considering the fate the whole MM series has shared) we can say, that 4 had a good strength of fantastic ideas. But that is because we can see what a great leap forward the game was and (as with oldies) we tend to overlook things that were wrong.

Still, when the game was released, it was a huge disappointment. The first release had tons of bugs (it froze after each battle on my system :/ ) and I can remember how I hated the game for quite some years. It was only after I bought Heroes 4 Complete after several years that I felt in love with this game. But that's due to the pacing and storytelling in campaigns, not the game quality overall.

I've played most HoM&M so far, and they all had overpowered stuff (which got corrected in the next version)

In one archers were too strong and an army of archers/ranged would kill anything but armies heavy on the strongest and fastest flyers. Then it was magic, up to the point you could go into a battle with some meatshields and magic your way to victory. It's a work in progress I guess.

Though the 9999 fairy army in.... 2 I think? was fun, as well as the 9999 peasants with the perfect necromancer

The difficulty imbalance was frustrating - I would spend weeks trying to gather an army... only for my hundreds of peasants to be wiped out by 3 mice or something :P .