It depends from the distribution of the skills of the unbalanced player
If he has low values in skills that are fast to train(i.e. JS-DR),the boost could be very useful;if he has low values in skills that are slow to train(i.e. OD-JR),this lack of skills could create you problems while you try to train them,for example a player with low OD could be foul prone and he could not have the complete training every week,so losing the effect of the boost,besides having a less useful player in the short time