Surely this just boils down to the ‘need’ for such a facility in the location that OP is talking about. Rural location unlikely, larger population centre more likely. If the objections are sensible, everybody has the right to oppose a planning application.
To those in the mental health sector (workers, not residents), you have my admiration but arguments based on checks and balances that “should” be in place wouldn’t convince me not to oppose such a planning application. Plenty of examples of things going wrong, which is almost a given when considering the lack of budget available for such care.
The point that philc makes of serious nutters on day release from prison is well made and shows that ignorance is bliss.