The biggest problem with varying flight times, I would think, would be the airplane's arrival window and "reservation" for the runway and terminal.
Planes in the air have a lot of variability.
On the ground, things quickly get complicated. Miss your landing window by 5 minutes, and you are left flying circles over the airport until they can find a window to fit you in.
Miss your gate time by 10 minutes, and simply getting across the tarmac to the gate, can be a problem, with congestion and ground crew availability.
Seems like if they go with TASAR to improve flight efficiency, they also need to automate the arrival and ground operations with some sort of adaptable neural net, overseen by skilled human controllers.