- What are the drawbacks of Heroes 3? (What didn't you like about it?)

If I'm being honest, I'd say:

Level 5 magic spells: Hypnosis and Mirror Image!?

Yes, there's a complete imbalance in magic in Heroes. It's especially noticeable at levels 5 and 4 – if you get Hypnosis or Summon Elemental (no matter which one) at level 5, while your opponent gets Teleportation or, at worst, Explosion or Flight, you really feel this imbalance...
Level 4 is also very uneven; it's frustrating when there isn't a single combat spell at level 2 – just espionage with "obstacle removal."