Problem is of-course that it's the instructor who is best fitted to judge what "good" is, and many students get frustrated by this inevitable difference in perception. (I think this works in any sphere, the oddments I've taught - varying from outdoor pursuits in my youth to test flying now, but I'm not a QFI to be fair, there have often been marked differences - in both directions - between my perception of ability and the perception of those I've taught).
I know a QFI who tried to send a student solo for months but the chap was so under-confident he kept refusing.
G