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For encoding it's always (end-start)*u"?":
>>> u"ää".encode("ascii", "replace")
'??'

But for decoding, it is neither nor:
>>> "\\Ux\\U".decode("unicode-escape", "replace")
u'\ufffd\ufffd'

i.e. a sequence of 5 illegal characters was replace by two 
replacement characters. This might mean that decoders can't 
collect all the illegal characters and call the callback 
once. They might have to call the callback for every single 
illegal byte sequence to get the old behaviour.

(It seems that this patch would be much, much simpler, if 
we only change the encoders)