Symmetra on final objective is so useless. The spawn is right there, she doesn't have time to set up turrets and her weapon is pretty bad.
Yea her strength comes from the first objective on Defend maps.
that initial choke point of each map has a bunch of tiny bottlenecks which she can spam her turrets on to. On top of that she can place a portal to get her team back in the fight way faster. Both of these strengths are greatly diminished once you get to the second objective tho.