The problem is that the tank unit that is best to use, right now, is workers. Melee soldiers don't actually have any use without improvements.Melee wood ants can not dig, transport eggs or build/upgrade tiles. You need workers to get to the surface and make/replenish melee ants.
They are strong, but this doesn't matter too much for a non-competitive game. The levels they appear in just need to be balanced for them.I think we must also keep in mind how overpowered this species is already. There's more than enough posts on this forum about it, in particular the rapid fire ones.