Mercy can only do so much.
If you know how to play Mercy, you know what she can do in any situation. Thus, yes. She is managable.
You can also change Mercy to pretty much any other name. But with Mercy specifically, you have so much less to worry about what she can do. Mercy just dies to headshots. Her heals can’t sustain them, she can’t take them. She’s either running, or pocketing. How complicated is that?