that's the point, you're supposed to use it on an unsuspecting group of enemies for maximum damage
that can be said about a number of specials though. problem is it's not as easy with pharah
if they know you're there before you ult you're probably already dead, wait for them to be distracted by your teammates
i have no problems getting kills w/o the ult it's just that in the thick of things its never as useful as junkrat's or reaper's alt where most of the heavy lifting is done for you.
Okay, for instance, I just played a game with Pharah. I got a kill before using her ult and killing 3 people, getting me play of the game. Now the only reason that it was so easily for me to get those 3 kills is because it of specific conditions like being indoors on a CP that i can predict the enemy's entrance from and the enemies in question weren't able to take much damage.
I still ended up with 4 gold medals and the ult helped (like 3 out of 27 kills) but Pharah's main utility is just her ability to keep people off balance. Idk it's a good ult but the conditions to actually do work with it is higher than other ults.