I don’t play genji, I took him maybe 10 times in all gametime from Overwatch 1 to 2 but here is my take on his ult.
It is not meant as a normal aggressive ult like diva where you just pop and then everything pops. Genjis is a tactical ult that you fire up when you team is trying to push into the enemy and doing damage but not kills. You come from the flank, pop the ult and dash-kill into the team, resetting your dash, you start swinging at someone squishy and insta dash, refilling it from kill, mostly now the heals are down. If time left you go for something else unless your team can could finish off because no more heal.
For me it seems its a very situational ult.