Blizzard stated Mercy is the purest healer and should all be about healing and mobility. Making her an off-healer contradicts that and doesnt make much sense for her. She is all about healing and helping other letting her shine in that role should be alright.
Rez was fine as her ult and even the issues it had could have been fixed with some tweaks. So yes her powerful heals and rez worked together very well without being an OPAF monster. That part came after the terrible rework the devs gave us and refuse to let go.
And even after the rework her healing wasnt the problem it was and still is valk and e-rez. Those two things need to go and if you ask me rez needs to go back to her ultimate in some form and valk can get deleted entirely.