Luckily, no one forces you to use it. I enjoy Mercy, when I don’t care about teammates, because she can fly - as it keeps all pesky flankers away. I usually play with Pharah, so no flankers can reach me up there.
I can rely on one player - my partner in duo. Anyone else, no dice. Even when Mercy had her old ult, I often used it in mid-air - descending too low is dangerous and puts me in reach of all kinds of enemies. Snipers or hitscan enemies can reach me, of course, but that can be avoided by using various buildings as cover.
Teammate dying with Mercy usually isn’t your fail, it’s their fail, as 50 hps doesn’t provide much room for bad dodging. Players often greatly overestimate sustain you can offer, and as result - pointless and sometimes stupid death.