I like Ezio where he stands right now and almost every hero in the game can be countered by some other hero. In the case for ezio, he can be countered by Arachna. It would be nice if soulstones could be accumulated but it isn't essential. You can still do really well with just a three star ezio for one dollar... I currently have ezio at Seven stars and it really isn't much of a difference. I feel the extra stars he gets tends to feed the other team with energy because he does so much damage quickly on his attacks and can back fire quickly if they have firehawk or Taurus on the enemy team when they use their ultimate.
I dont think it works that way. I'm not sure about the game formulas but it feels like you feed the enemy more when you attack multiple times for lower damage. The higher damage you do the more likely it is that their HP bar drops faster than their energy fills. Ezio will not kill anyone by himself before he ults so he will give a bit if energy to some heroes, nothing too serious. Also he is pretty good at killing firehawk. The second he gets his ult firehawk is gone, unless there is another opponent with less hp but there is a good chance it will be firehawk