The cast time Mercy. It worked so well because unlike early Valkyrie, there was actual value to her ult. You pop into ult, get a safe rez out, and support the team from afar. Because the non-Valk rez was so risky, getting that safety curtain from Valk made it such that you had something to work to, there was reason to even bother caring about whether you have ult or not. In the current PTR, ult means fuck all and so why particularly try to work towards it or get crucial ult building heals when you are still in a large amount of danger while rezing, probably even more because she's far from subtle looking when in Valk so it's arguably even easier for a Widow to quick see her and go for a snipe.
The way she is in the non-PTR version right now is great.