Keep jumping and run around the enemy, she's actually pretty hard to hit and she doesn't even have to aim. When I take her I always manage to kill a surprising amount of people, especially on Hollywood defence since it has a lot of places to hide.
Yup, you gotta pick your spots and try to grab stragglers before they make it back to their group (too many times I've been on somebody's tail and was so focused on keep the beam on them, that I end up following them all the way back to the rest of their team... dumb dumb dumb), but Symmetra is a beast when you know where to flank and catch people by surprise. My favorite trick is to hide up on higher ground and, when a group of enemies comes by, jump right behind the last one - usually the medic - and hit them from behind. Often I can get a kill off and hoof it before they even know what happened.
Oh, and on behalf of all Symmetra players: fuck Junkrat's passive skill. Fuck it so, so much. I always know it's coming and yet I can never get out of the blast radius in time.
I believe she still has the overall highest winrate on PC.
I saw somebody explain that somewhere else, I think on Reddit: Symmetra is a character you switch away from in the course of the match, and usually never switch TO. Most Symmetra players, myself included, will usually pick someone else when the game isn't going our way, and the enemy is pushing too hard too fast. And since the game counts the last character you're playing as as the one who "lost," then that's why Symmetra's win rate is probably artifically high.