Valk personally ensures my safety. It is pretty difficult to kill Mercy during Valk. It also helps for me to spot / cover heals for more of my team with the beams. The infinite ammo is good, especially when there is a free moment to fight a widow that might be giving the team trouble.
Overall between increased mobility, health regen, distance when healing, chain beams, and infinite ammo, the ult provides strength in its versatility.
This does not mean I think it works well as an ult (or that I think mass rez should return in case people jump on my case about that). Valk to me seems like a long CD ability that was placed as an ult.
Nearly every ult in the game has infinite gratification on some level, there’s a very clear reason to use it and it makes you feel powerful. There are clear defined moments when you should or should not ult as other characters. Mercy’s momemts are not always clear which makes it feel unsatisfying as an ult.
Personally, I’d tone it down and move it as a long CD ability and replace the ult with something else. But that’s just me.
The range and mobility the most. If I need to, I can go as high as the sky allows and still heal / damage boost. Or I can be far away and low to the ground, out of LOS of my enemy while the chain beams still get my allies around the corner.
I like both her mass rez and valk for different reasons. As an ultimate, valk however feels a bit of an oddity compared to other ults.
I’ve come up with ideas in the past, however I don’t have one of the top of my head that I feel is worthy writing out, would need to flesh it out more.
That being said, while I do think she could and should be changed at some point in the future to have more clearly defined gameplay, I don’t think she’s currently in a bad spot. I think there are other characters who desperately need blizz to focus on them more before Mercy, unfortunately. Doomfist being my top pick followed by Brig (whether you’re a fan of hers or not, is anyone happy with how she is right now?), and Sym.