Yes, I think they are too strong:
- Ninjas have the same power as the strongest costumed heroes, but ninjas have also a family bonus, unlike costumed ones.
- And it seems ninjas have a super-fast mana speed.
- Moreover, ninjas “grow” faster and cheaper than costumed heroes.
This all makes them obviously stronger than any other hero.
Why did ninjas get so many benefits?
In my opinion, ninjas need to be balanced with heroes from other families, otherwise the disbalance will be very unpleasant.