It seems to me siddhi-s might well speed up the ripening of karma. So, what's the result of practising them could depend on the nature of ones praarabdha-karma (karma ready to be experienced, or stuff?). I believe it could, for "non-yogis", be black (kRSNa), white (shukla) or "grey"??
karmaashuklaakRSNaM yoginaH, tri-vidham itareSaam. (YS IV 7 ?) A yogi's karma is neither black (non-black: a-kRSNa) nor white (non-white: ashukla), for others it's of three kinds (black, white, grey??): One guy on my siddhis-course got such a severe neurological condition, that they had to give up their profession and learn a new one. I'm not sure whether they had that condition already before the course, and it got worse during it.
