as for medic, it would be an insanely overpowered class if they had jets, same with demolator
Thats kinda, the point.
So I can tune the jets so that is isn't OP.
Lets assume Pyro Jets have a duration of 1
Then the Medic's would be around 0.45 Just enough so you can jump over, avoid, without being extremely OP.
As for the Demoman.
You just can't set up a getaway in time.
The Charge-N-Targe provided the speed boost the class needed.
But you're right jets would be over powered.