It's just that if they make a mistake they'll never admit it, and they won't take responsibility to right what they've wronged. Or they'll use cheap materials and cut a few corners with their techniques.
密醫做美容，其實「刀下工夫」未必輸給正牌醫生，只是他們做壞了往往不肯承認、不負責善後；或是用較便宜的素材、較偷工減料的手法蒙混過關。