To me thorn is fine because of what you stated there are guns that keep it in check if used correctly. Like red death in my experience, I can get a second shot out of red death about the same time thorn does or maybe 2sec after thorn fires it's second shot now if both thorn shots where headshots the dot will kill me but i used red death correctly where both shots has the third shot from both bursts land on the head it should be a kill and red deaths healing perk will save me from the dot of thorn but again this is my experience. Just seems people don't know how or just aren't good enough to counter it