Honestly, I think Mercy is just inherently boring. You wouldn’t suck the fun out of most heroes by changing their ults. If you gave Ana some other ult, even a lame one, she’d still be almost as fun to play. Same with Moira or Lucio. With Mercy changing the ult ruined the fun people were having, and I think the reason is her base playstyle really isn’t very fun.
Other healers have a lot more to do - Ana, Lucio, and Zenyatta are constantly shooting, Lucio also has his complex movement, and Moira has to constantly juggle attacking and healing. Most of them also have a lot of abilities to use. Brigitte has an aggressive playstyle that requires a lot of risk taking. With Mercy, there is an awful lot of unengaging beaming. Sure, you have to switch damage boost and heal, but that’s pretty easy to figure out (generally, if healing partner injured, heal, else damage boost). She has her movement, which is probably the high point of her design fun-wise, but that’s largely it. Without a big flashy ult to inject some color, Mercy mains are just starting to see the deficiencies inherent in her design, which are the reason everyone else has been calling her boring all along.