Rarely, do good critics make better artist. ATM Ben Jonson, the poet/critic is the only example I can think of, but that is more of a tie than him being better at one or the other.
Siskel had godawful shitty taste in Hollywood palaver and thus couldn't be expected to do any better himself.
OTOH, Charlie Brooker is a fantastically vicious UK media critic and social commenter. He also makes superlative TV, like Dead Set and Black Mirror.
The 'conflict-of-interest' canard is an excuse by and for those who can't but presume to opine on whose who can.