I don’t think Res is the issue.
She’s strong because she has unrivaled mobility, great healing potential, and survivability.
High impact resurrects don’t happen that often in my games; people are usually very vigilant about those. And when it does happen, it’s usually because the team isn’t in a position to punish, which generally means they’re already losing the fight (unless it’s res’ing an early pick before the team fight).
She’s just reliable. So much so that the unique properties of other healers are overshadowed by her simplicity and sheer coverage. She’s the only viable healer for solo heals and she’s always involved in multi-healer comps. The only thing she doesn’t have that’s extremely valuable is an anti-ult.