You can position your damage boost more reliably without forcing teammates to push into unfavorable situations to keep their buff. You yourself are nearly immortal and have more mobility to make a play off of your own ultimate.
The user can travel faster with it to escape/return to a fight/get to a distant rez/etc.
It’s benefits (hps/DB) can be moved to wherever it’s needed once activated.
It provided unlimited ammo to the user.
It attaches to a teammate, the teammate doesn’t have to attach to it.
It provides flight and a stable hover to the user.
It can be cast in one place, but applied a distance away.
The user can skirt around some sections of the map others are not able to.
Just to name a few.
It’s kind of an apples and oranges thing, IMHO.
You get to play mercy and can actually fly around in all directions.
Unlike budget boots that skipped the landing lessons
Covers a vast amount more area and makes Mercy much harder to kill than Juno’s ult does for her.
Tradeoff is lower numbers and Mercy not benefitting from the numbers herself in exchange for being tremendously more versatile
You can control who gets the most healing or damage boosts (i typically just heal). Juno’s Ult is on a predestined path with no control from Juno after its been activated (yet!).
free flight and you become really hard to hit for stalling potential, ability to get cheeky easier res off, ability to hide to get value with it whereas juno has to follow the ult
it’s not a great ult but there are some advantages
You actually control it… >->