I take it KERS is "Kinetic Energy Recovery System" which takes energy from your momentum and stores it temporarily, also offering a braking effect. With the intention to use this stored energy for subsequent acceleration.
A spinning flywheel on the front wheel may have significant precession effects when spun up. If the rider tries to turn left or right, and the flywheel is co-planar with the rim, then the wheel will experience a rotational force around the long axis of the bike.
In short the front wheel will try and lie on its side.
This would prove dangerous if unexpected by the rider, and the scale of the force would vary with how much rotational energy the flywheel has stored.
A flywheel on the back wheel would experience similar forces on cornering, but the back wheel follows the turn rather than being moved to create the turn.
Downsides to a rear KERS, the wheel is already busy with drive, and often has disk brakes too. Adding something inside the rear wheel will increase the weight where we don't want it, and rear wheel braking is the least useful sort of braking.
So, added weight, added complexity, added adrenaline when the bike kicks sideways, and minimal benefit.