I’d rather have constant 50hps than “sometimes makes shots” 80hps. I frequently see complaints about Mercy’s potential output, but, in the right hands, her actual practical output is consistently much higher than the others. I can count on one hand the number of times I’ve been outhealed by another support when I’m on Mercy even after the 50 hps nerf.
As far as Valk goes, here’s a link to a post I made a few weeks ago regarding what Valkyrie can actually do (with video evidence) when it’s used as more than a spectator mode.