Scott Johnson was a pretty naff head coach when he was with the USA Eagles. He had just 6 games in charge, won 1 on them (against Uruguay), and lost the other 5 (and didn't come within 7 in any of them not even against Tier 2 nations Canada and Japan, and was tonked by England and Ireland A). And to finish off his time in charge of the USA, he took along their most impressive young player named Hanno Dirksen and brought him to the Ospreys where he now refuses to play for them and is lost to USA rugby for good. So from the USA's point of view, he was an utterly shi*t head coach.
His record with Wales as caretaker head coach is also played 3, lost 2, and one draw at home to Italy. So in total he has won one game as national head coach. Some in Wales weren't fans of his time with the Ospreys either and many consider his tenure a failure, so it is debatable as to whether he can be any more than an a assistant coach.
His finest moment as a coach was during the 2001 Lions tour, where he overheard the Lions lineout calls as water boy on the touchline and relayed them to Justin Harrison who then nicked the Lions lineout as they were trying to score in the dying minutes.