Overwatch is unfortunately not balanced very well.
On some maps and some game modes, Bastion and Torj are complete OP, on others, they are nowhere near as good. Some characters /ultimates/ are mimicked by others timed abilities (Like Widowmakers ultimate).
It's still fun, but a lot of being good at the game is knowing who to use on what maps in what modes.
As for healers, Mercy is the strongest direct healing but the mechanic to switch between support and offense is somewhat clunky. Zenyatta has the second lowest healing (Behind Sym) while Lucio is the best all around with damage, healing, shield support, etc.
On some teams, Lucio's AoE heal as low as it is is perfectly fine, on other teams you need Mercy's more powerful heals. It really just depends a lot.
Overall, the game is fun... unless you get stuck on a team with twits who don't know how to stop the platform/escort, or fight on the objective.