If you play mercy properly and not just Rez one person and let the rest of your team die, that's a shitty mercy. A good proper mercy can keep her whole team alive for a majority of the match if they stay close, have a shield hero, stay behind him and work together to get shit done. I never understood the problem with a Rez ability. Yeah okay the ult making it a 10 second wait to Rez again is bullshit, but it's a useful ability that is better than a 5 man Rez all at once. Especially because once she uses the ult she can help push the payload at the end more, a time where the defending teams use bastion, ,junkrat, pharah, or torb (mainly his turret) who can cause problems. Mercy can then get behind them and hit them, forcing the attention to her and not the ones pushing the payload. Hell to counter mercy very easily use fucking sombra's hack ability. You may die but at least you stop her from rezzing a rein, or bastion, or reaper, and allows your team to kill her and the others. Use strategy and teamwork and good team comp, and you wouldn't have a problem with mercy.
