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

In my opinion, in this type of game, unit defense should be a permanent skill that decreases in a proportional percentage to the number of cells traversed...
The first strike advantage is too unbalanced.

Regarding the changing defense – I would call this typical grinding. Similar annoying solutions were in Heroes of Might and Magic IV – and who plays it now?
Regarding the first strike – when it was removed in the fourth game, all sorts of fans of the third game were very unhappy and criticized the fourth game, although it is an excellent game. But you also need to know how to use the first strike, otherwise it won't save you from defeat.