I was doing some reading on Howl of the North and was noticing that people were suggesting that Howl might not stack with Overwhelming Crit and I was also curious to see if it was stacking with Devastating Crit from LD, so I decided to do some testing.

Weapon: Hewer
Buffs: only Skaldic, no power attack

Criticals with LD and Howl: 416-433
Criticals with LD and no Howl: 329-359
Criticals with Fatesinger and Howl: 329-335
Criticals with Fatesinger and no Howl: 249-269

It appears that Howl of the North stacks with both Overwhelming Crit and Devastating Crit.
If there is some issue to my testing you're welcome to let me know or do your own testing. It wasn't the most extensive, I only went for around 10 crits per test, but I'm satisfied with the results.

Finally, one thing I noticed when doing these tests is that even if the Howl icon in the hotbar is active, that doesn't necessarily mean that Howl is active. The icon has to show up on the buff bar at the top of the screen for it to be active.
It seems that on area transitions howl will be active on the hotbar, but won't be active on the buff bar.