As a non-Mercy player, I prefer Valk to Rez. I didn’t like losing my healer for a team fight so she could hide and intentionally let her team die. I also didn’t like the immunity Mercy instantly got when she hit Q. Valk feels more fair.
I also like the AoE healing/damage boost from an ulting Mercy on my team. It’s like a better version of Orisa’s ultimate.