The Legend of Korra

Well, he wasn't able to take out Amon, so it shows that blood bending has some weakness, although we're not sure what at this point. Amon probably knows how to counter it (which everyone in the trial obviously didn't.)

Edit: As an afterthought, Combustion Man doesn't break any "established bending rules?"

Hell you're kind of lop-siding it. You're saying that undiscovered knowledge doesn't break "established bending rules," yet you declare massive blood-bending outside the full moon "breaking them" while it could be very well that it was just undiscovered knowledge. I mean I agree it'll be very stupid if your average waterbender could do it and there's no counter, but we've already seen two ways you could break it, not to mention the two people who could do it can't bend anymore, so it should be fine.

As we know nothing really about Amon, there could be a million and a half reasons to explain his resistance to Bloodbending. Half Mechanical? Mostly Spirit? Bloodbender himself? We just don't know enough about him to explain that course of events.

Combustion Man certainly had unique Firebending, but he wasn't able to take out over a hundred people at once...or in a small amount of time...or at all (from what we saw, at least). Plus, his abilities were new, not a concept already given to us and then wildly thrown out of control to create a far too powerful enemy. Bloodbending isn't a new concept. When it was, it was limited, and shown to not be as powerful as it suddenly is. Combustion Man's abilities were introduced how they were, and even then they were nothing more than focused Firebending. It's kind of like Combustion Man was a Firebending Bumi, able to use his face as a Bending form. "Hey you know everything we told you about Bloodbending? It suddenly doesn't apply anymore because we need an evil SOB."

Undiscovered Knowledge doesn't break "established bending rules" because never at one point did they say that the Four Bending Arts were the only Bending Arts. Hell, it was essentially a lost Art, as explained by the Lion Turtle, "In the times before the Avatar..." How many books, games, shows, movies, etc. have added some "lost power" at the near end of the story? It's no different here.