In terms of score Lifetowers are bad, very bad. But i think they have a place, as they allow not so good players to reach later waves, instead of getting eliminated like wave 20-30 on hard. By that they get more experience playing the game. I remember eletd in starcraft 2 where it was about getting most frog kills, there life towers were quite op, as they allowed you to build up quite some hp-buffer.
edit: forgot to say: nature towers are too op, atm (as only towers and slow towers in general are underpowered, in terms of getting a good 900k+ score)