You kind of went off in another direction there.
What is pedaling smoothly, then? Pedaling in a circle? Delivering power fluidly? Because you're not delivering power in a full circle, obviously, and you're not even pulling up on the pedal, you're simply unweighting. So all of that pedaling is essentially stomping as power is produced on the downforce.

I think you just made up the improved endurance part. An efficient muscle sequence? That's only available on rollers?!
So does a trainer.
So does a trianer.
So does a trainer.
Disagree. Groups and traffic dictate vastly more complicated and substantial changes in direction, as well as reaction times.
Balance is always utilized on a bike, regardless of riding on rollers or not.

How does a trainer harm balance? Within 1 second of pushing off outside you've regained balance.

Rollers are not this magical training tool that everyone always tries to make them out to be. They're simply a way to ride indoors. Just like a trainer, except for the falling off part and the potential decreased ability to complete higher power workout if you don't have extra resistance. Plus, ever tried taking turns in a group on Zwift on rollers? "Fun".