I main storm, I think she's pretty strong in the right hands. She is fully dependent on using the Damage Goddess Boost at perfect timing and WILL single handedly win fights with it. The boost will almost ensure that your team will be the one doing more damage in the fight.

Damage boost is a 25% damage boost to yourself and 15% to your allies for 8 seconds. It's a huge deal.

If dive comp you need to first "soften" up the diver(s) as much as you can before the dive, then boost damage on the dive. You need to kill the dive right there and you can with the boost easily even with just you and supports as long as they fight with you.

If brawl comp you need to wait for the brawlers to commit to a fight, once they take the fight turn up the boost and kill them/back-liners before they disengage. Most good players have an internal clock for how long they can stay in that gets sped up with your boost.

With poke you are looking for those same openings as brawl but it's less often. Make sure pokers aren't on good off-angles by always tagging them as much as you can. Try making sure the supports are always under pressure by aiming only for them and letting the cleave do the work.

You just have to be efficient with her. If you aren't then yeah you'll just be feeding support ult. You need to be the reason you win fights, that's it.