The reason I am perfectly fine this being a OP exotic (It isn't even OP) is that you have to EARN it. Other Exotics like Hawkmoon are RNG based an may have no actual skill involved in getting them. An example being the Last person in a Crucible match getting Ice Breaker, no work or earning involved. With Thorn you not only have to be RNG lucky to get the Bounty but go through 5 Or so Trials involving nearly every aspect of the game. Thorn therefore, with one of the hardest exotic bounties, is justified in being powerful and getting a Generous Buff.