The biggest problem is power, like few people above me said. Fast heroes should be good, but not OP, so you would focus on them, but you will survive their attacks - good examples: Lianna, Alice
Average heroes should be, well, average. Their attacks should be more powerful, but nothing you should be REALLY scary about. Good examples: Malicna, Glenda
Slow heroes, however, should be powerful, and people should be afraid of activating them. These heroes shouldn’t be focus on early part of raid, but definitelly pain if you forget about them. Good examples: Alfrike, Mother North
Sadly, heroes in game are pretty unbalanced. And being unbalanced doesn’t come from stats, but from mana speed.
Fast heroes are too dewastating - examples: Frida, Finley. Sometimes raid is over after their first attack - it shouldn’t be possible for heroes that are first to strike.
Average heroes are usually good balanced, however their main issue is because of that they are usually weakier than Fast heroes.
Slow heroes… Slow heroes, like I said, should be round changers. But unusally they aren’t. Like Noor or Bertila. The biggest problem however are old slow heroes. When they were introduced, they were made Slow to balance their stats. But times changed, and they are not that good anymore. The best examples here are Justice and Boss Wolf. They were monsters at their time. Today? Not so much.