one Sentinel has about 50% uptime on one target in a long fight. So making it perma in a gm or av fight is definitely an issue, past the first 45 seconds or so. Depends on how hard stuff you’re fighting of course.
It’s not for minions. Using it on a minion is a waste.
The Debuffs ignore level differences, so it’s stronger than you might think, and the debuff and mez resistance debuffs are also handier than you might expect.
Considering it’s an auto hit def and res debuff with no end cost and no activation time, it’s about as powerful as would be balanced. Keep in mind the old Opportunity debuff also did not stack, only the small passive debuff. That was replaced by the base damage increase, so losing the stacking part is not really a problem.