If you think parkour seems overpowered, it’s only because parkour is totally overpowered. I have a friend that likes to scramble up the sides of buildings, and it is a remarkable pain in the ass to keep up with him. I guess I know how the zombies feel.
But yeah, for balance in game, it might make sense to add a chance of failure. Maybe have a general “athletics” skill that affects running, climbing, etc. Parkour can still be a trait that does what it does, and also gives a boost to athletics. At higher levels, players can just do parkour stuff using the movement mechanics for sprinting, jumping and climbing without having to train in a specific style or skill.
Just don’t try to jump that gap between buildings before you’re ready, or on a rainy day when it’s slick. Or if you’re hurt. Or if you’re stressed.
Or if you’re not spiderman.