What complicates breaking in a curve while riding a motorcycle?

The short answer is that braking in a bend is best avoided if possible. That said, there will be unforseen circumstances which make such braking desirable if an accident is to be avoided.

The complication of braking or accelerating through a bend arises because of the gyroscope effect where changes in wheel speed seek to force the bike upright whilst it is banked over. This make the bike difficult to control. In addition, whilst the bike in banked over, the tyre contact patch with the road is smaller that whilst upright. Braking or accelerating hard in a bend runs the risk of skidding and the bike sliding away as the tyres lose grip. Therefore braking through a bend is best done carefully, combining both front and rear brakes with engine braking and avoiding grabbing a fistful of front brake, which will almost certainly result in a front wheel skid and loss of control.