be a double-edged sword
To be something that can be both beneficial and problematic. Going back to school was a double-edged sword for Pam. On the one hand, it widened her career prospects, but, on the other hand, she was in a lot of debt when she graduated.
be a double-edged ˈsword/ˈweapon
be something that has both advantages and disadvantages: This new ‘miracle diet’ is a double-edged sword — it’ll make you lose weight fast but you may have some unpleasant side effects.