The clearest way of putting the problem (in my mind) in this: The literal meaning of 'all but' is 'everything except,' but that is not what is meant. Consider:
My boss' constant nagging made the job all but impossible.
'Everything except' suggests that the nagging made the job everything except impossible - but that is clearly not what is meant. 'Almost' or 'virtually' seems to fit better, and this is why 'all but' is a poor expression - its literal meaning is somewhat at odds with what is meant. We should 'eschew' it - don't you think?