In the first form, a fault is denied, then an exception particle such as *illā* or *ghayr* introduces something that is in fact praiseworthy. The apparent turn toward criticism becomes further commendation.
The hearer is led to expect an actual fault, but the subsequent words defeat that expectation and reinforce the praise.
The alleged exception must be a genuine excellence in the intended context. If it is genuinely defective, the construction no longer has this rhetorical effect.