Could bladder slings become dangerous?

Yes: If they erode into bladder or urethra or become infected.

Highly unlikely: It is very rare for a mesh bladder sling to become dangerous. However, there have been rare cases of the sling "eroding" into the urethra or the bladder -- this would be a serious complication that requires surgery. In contrast, a mesh exposure in the vagina can be fixed easily and generally doesn't cause continued problems.