Many answers here are pretty good, also many of them are uncorteous in the least of cases and plain insulting, if not offensive in the worst.
Having read on the original question that the phrase would be used to offer advise or criticism to a relative, and all things being equal, you may wish to be or stay on good terms with them, a milder but no less precise word is required, and I suggest :
- showing little skill.
- Not carried out.
Merriam Webster further adds: " lacking talent, poise, grace, or achievement"
Which seems befitting, since having practiced for a lifetime and being a person who probably wasn't negligent in his professional development and education, this word conveys the lack of something that would make him recognizable or memorable above the ordinary and everyday, but implying that whatever is missing is not his own fault or has yet to come, in order to be accomplished.
Using your example:
Look at Uncle. Practised music all his life and still not very good at
it: he is an unaccomplished musician still.
Let's never forget that language is a tool for connecting with people, and people have feelings, specially about their livelihood and career choices. Advise will be more readily accepted and even appreciated if it is delivered with kind words.
Imagine someone offering you advice in the form:
[someone's_name_here] Look at you, all your life
[occupation] and you are, to be honest, still not
very good at it! You are an
[unkind_suggestion], and I think you
[unkind_suggestion] with some of the accurate, witty and patently correct answers in this thread.
Most likely, your party will stop listening before "... and I think you should....." failing the whole purpose of communicating with someone else, even if your advise was good, pertinent, clear, pragmatic, useful, realistic and profitable.
You would have just wasted your time, lost karma and brownie points with those who disagree with you and probably this person's friendship. Not a good deal in the least.,