Maybe... BUT!!!
Don't forget, Heroes 3 was made by EXPERTS who knew this game inside and out, and they worked on it for as long as they needed. Heroes 5 was made by people who weren't particularly knowledgeable about it, and they had a LIMITED amount of time.
By the way, there isn't a single bug in Heroes 5 related to creatures (but there was a "Silver Pegasus bug" in Heroes 3).