I think there's one aspect of Hagg's game that doesn't show up in advanced metrics - shot blocking.
Given that the Islanders lead the league in shots blocked, at least one top HC believes it has significant value.
Block a shot and it still counts against your Corsi and doesn't help your xGA.
But it will reduce goals scored against you when you're on the ice, ask JVR.
Provorov is the other D-man who blocks shots at a high rate, followed by Braun.
Pitlick is the leader among forwards.
As far as I'm aware, xGF/GA stats use Fenwick, not Corsi. So, actually, a blocked shot
does reward the player for blocking it by counting as a non-event. So, someone like Hagg generally performs slightly better by expected goal metrics, though for a couple other reasons as well, but it's just a matter of degrees from awful.
Blocked shots are messy. If you peruse the leaders in a given season -- and I'm using rate stats, not cumulative -- you have good and bad players next to each other, with not a ton of correlation. Enlarge the sample size, and there's probably negative correlation, as regards a player performing well in the possession metrics. Big Bad Sanheim was top 20 in blocks/60 last year, and he's dead last on the Flyers this year, right alongside Niskanen.
That doesn't mean blocked shots don't have some intrinsic value, hard as it is to parse out. But perhaps a player (we'll stick to defensemen) is passive in defensive coverage and cheats for clogging the lane over attacking for a change of possession (I think you know who I'm referencing)......does that legitimize the block as the best outcome? Getting hemmed in and blocking shots is not a
net value gain. In the end, blocked shots do get properly accounted for in the quality stats.