Not sure why Gardiner gets so much abuse.
He has been a bit of a slow learner and his mistakes can be glaring ... however, his numbers are pretty good.

He is 30th in points (d-men) and 8th in plus minus (d-men).
He averages over 21 minutes per game which is 68th (d-men) in the league.

Considering there are 284 d-men listed on the NHL web site ... not bad numbers at all.