After exclusively using my Vex and Vision of Confluence in Crucible, I think the only item potentially OP about the Thorn is its range.
The Thorn is an exotic with the hardest bounty. It should win nearly every gun fight it enters... Unless, it meets another hard to acquire exotic in that exotic's sweet spot.
The Thorn should not always win against a Vex at mid to mid / long range and lose as much as it wins versus a VoC at mid/ long range.
In my tests, I won my share of encounters vs the Thorn at these distances but was amazed by the Thorn's range.
I would only nerf the range.
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It's well known that thorn isn't really OP. It's a hand gun and not even as powerful as some of the ones you get as legendaries. It's two main points of contention are that people who get hit by it panic and complain because of the weak ass poison effect. It has final round which people forget so it sometimes wins battles that it technically would have lost.