Ultimately with Mercy there were two kits which had massive issues:
Old Mercy: Without an E ability, Mercy was limited to flying around and healing teammates along with buffing a single DPS dealer (Pharah, or a Precision hero such as Widowmaker, McCree, and now Ashe) meaning Mercy is normally very boring to play. Mass rez, in the state it was in, only was beneficial to your team if nearly everyone died. This meant that, while not universal, there was some mechanical encouragement to hide and rez, which caused problems for everyone involved. Mass Rez in this state was too unreliable. Mercy 1.0 needed an E ability and some tweaks to Mass Rez to benefit LIVING team mates and mechanically prevented hide and rez.
Valkyrie Mercy: Rez on Cooldown is a frustrating ability to balance, since initially Mercy could Rez for free, but now puts Mercy into an exposed state. Valkyrie was gutted thoroughly to the point it is one of the most lackluster ults in the game. Most of Mercy’s interactivity was removed through nerfs, resulting in her being, again, boring to play.
I do think Blizzard needs to go back to the drawing board a bit with Mercy. Historically speaking, Mercy metas were the most diverse for tank and dps roles, but being forced to play a fairly simple and uninteractive hero causes a drain effect to support players who want to play something else. Maybe a redesign of Mercy 1.0 where she has an E which allows her to remove debuffs (and mechanically similar to armor pack) and Mass rez giving this cleanse and heal burst to her living teammates in exchange for a line of sight check.