It’s the main thing holding Mercy back. Without Valkyrie, she could have an actual ultimate with Mass Res on Q, or some other Ultimate that is actually engaging, impactful, and rewarding to play. Valkyrie is hated both by people who play Mercy, as well as people who don’t play her, and since the rework Mercy was made OP for 5 months straight - Being very unfun to play against. Afterwards she was given 12+ nerfs, losing what little impact she had in the fight, and resorting to being a glorified spectator cam - besides being one of the most unrewarding ults in the game, it’s very unfun to play with.
Overall, I believe the rework has been a huge failure, and Blizzard would be doing everyone a favor with it’s removal, a Revert of Mass Res + Tweaks / Another Rework, and a new E ability. Something many people wanted since day one. 
“This was a huge success™®”
~Sincerely Yours xoxo,
a Lover of True, Fair, and Fun Balance.
xavvypls 