It's not that it's less believable, it's throwing these new things into the mix without any apparent reason or justification for them. Blood bending, when it was first brought up, was this mythical super-rare only-works-on-a-full-moon special case thing that had a whole episode dedicated to it. Korra book 1 comes around and, oh, yeah, bloodbending's just a thing now, some people can do it so casually they can even pass for non-benders. By the time we hit book 3 we're now just introducing completely new kinds of bending out of nowhere with no one seeming in the least surprised about it. It just feels progressively as if the writers are introducing new powers as they feel like it as opposed to what's important to the plot/the world of the story.
"Hey, what if one of them could bend lava?"
"What? How? What element does that even fall under? Why?"
"Because it's cool!"
"Welp, I'm convinced."
Don't even get me started on "I have an eye in my forehead that causes explosions because Reasons" lady...
Because that wasn't done during Book 3 of AtLA? Sparky sparky boom boom man? It's not like it's probably a specific clan that's hereditary or something like a lot of other specific abilities. Or that Heat bending isn't an established thing. Really? You're using her to support your bashing of new styles?
There are a LOT of things that weren't necessarily covered in AtLA and so LoK is getting to explore it. It's not like Lavabending wasn't a thing! It might have previously been an Avatar ability but with the tribes intermingling a lot for the first time in a century, it's bound to happen. As I explained it's no different than a water bender learning how to bend Ice or Plants. It's changing a state of matter which has already been established. It just takes a stronger bender to do it/think of it. It was practically thrown in Bolin's face to learn how to do it because he'd otherwise be dead. I'm sure there's a similar backstory for Ghazan.
To be honest, no one's been surprised about it because the more inventive/torturous fanbase has already thought of it.
Also, he wasn't doing it so casually he could pass for a non-bender. He wasn't using bending at any other point in time or advertising that he was a bender. Amon was from a special case, from a family of powerful benders. Yes blood bending was established as a "rare" ability because no one taught the one water bender from the southern water tribe that it exists and the one blood bender other than Katara that we meet probably wasn't exactly the most powerful.
So we have a skill that isn't taught/is probably discouraged in the Northern Water Tribe. Katara isn't about to use it because she hates it. She has also been told it is only possible during the full moon. If you're told something is only possible during a specific time, you are less likely to attempt it at any other time. If you are a powerful bender specifically trained to blood bend when it isn't the full moon (because there was an entire training explanation) you're going to be able to do it. It make sense. You might have to think a bit. But AtLA made you think a bit too. Why shouldn't LoK?