To be truthful even if you don’t get anyone with mccree’s ult(if they’re on the point) it’s still good as you’re making your team have that extra push if they’re attacking or defending, like someone said up top, even if you get one kill also it’s still a good ult. when aiming always try aim for the neck as you’ll most likely hit more headshots than actually aiming for the head, don’t constantly spam the trigger, take your time(but not too long) and basically track them.
When using flash(especially against a Gengu) aim way above his head when deflecting as long as he isn’t right in your face as you should still stun him. Try to bait them to use their abilities(but don’t use your flash instantly). If you wanna use FTH by all means do it, many will hate you for it but it’s there for use, I mainly use it on big targets or shields but you just do you bud.
Also when stunning someone (other than tracer and zen*i think*) try shooting them before you flash to get a better advantage for the next shot after(headshot obviously). For his ult try to always gain high ground(if you can sneak behind without anyone knowing and know your team is atleast winning or holding it should be fine) then when you’re behind zone in the deadeye, if possible try to always get the supports so your team will have that extra advantage.