It would seem to me that there are several variables that would determine the ability to effectively use Aikido in such fashion as to protect oneself while not unduly harming one's attacker.
The primary one that comes to mind is the prescence of weapons- this will increase the magnitude of hazard to both parties. While I have seen individuals with an excess of fifteen years training who seem capable of handling armed attackers without inflicting structural damage, I would personally fear for the safety of either or both parties in an altercation where one was armed, and the other had less than ten years experience. I would also look at the type of training which the defender had been exposed to. I have trained with some individuals who are police officers and prison guards, in a dojo where the training is very pragmatic i.e: there is repetition of only one or two techniques, or variations of a technique, within a 90 minute class. An emphasis is placed on correct body placement to minimise exposure to subsequent attacks, understanding of anatomical structure and awareness of the role of atemi. Conversely, I have also trained with individuals who emphasise energy practice and a more flowing approach in which strikes and structure locks are less prevalent. Both have their adavantages and disadvantages. For an individual who is likely to need to employ Aikido technique to counteract a serious attack within a shorter time of beginning their training, I would suspect that the first training style might prove more efficacious, though this may also be a situation in which the attcker may suffer some injury or discomfort in order to prevent serious harm to the defender. That is only my opinion, however, and I welcome hearing from others on this question.