Whether it was legal- Unless there was no sign the specifically indicates that a U-turn is no legal, it is generally not against the law.
One example of a U-turn that can be at fault is if a U-turn was made illegally from the middle left and there was no sign prohibiting the driver.
Safety of a U-Turn -Things to consider:
-Was it made in a tight turn
-Did the driver not wait for a big enough gap to pass
-Stop or Slowing
-Not going with flow of the law.