What constitutes realism?
Risk, danger, threat, unpredictability. There must be some concern that you may be harmed.
Attacks need to be unrehearsed.
The assailant can punch, kick, grapple.
They can be armed.
They can have friends.
The key feature is this: the attack must be vigorous enough and threatening enough to trigger the defender's nervous system.
It is not necessary to injure anyone.
But the attacks must feel serious and create a genuine emotional, psychological and biological response in the defender.