It's murkier even than that. The new Marvel comic has one of the holocrons collected by a Hutt character mention "the Hundred-Year Darkness", which is from the EU as another term for what would later be known as the Second Great (Jedi) Schism. In the EU, those exiled/rage-quit fallen Jedi were the ones who found the Sith species, intermarried with them, and led to the Dark Lords of the Sith and the Sith Empire. Now the term is part of the canon, but the rest of it...? How far can we extrapolate, if at all? It's agonizing to have one bit of a story, but not know how much of the rest is still valid. *heh*
And that's just one example. They've peppered the new canon with a dropped name here, a mentioned reference there, a cameo appearance over there... but we can't take any more from it than just that specific datum until they include more. The general trend, though, is that they're working as fast as they can to organically include the good stuff, while quietly ignoring the... less-good stuff, from the distant past up through the end of ROTJ. It's only the stuff from that point on that's gone gone. Plus the lingering question of what that means for people, ships, etc., from that era that had activities that took place earlier in the EU -- notably Mara Jade. Thanks to Katie Lucas making Maul from Dathomir, now, instead of his old EU origin and training, those are now no longer Sith tattoos, so that's out. I think the Eyes of the Sith is still a thing, though. Which means, again, that Kylo and Snoke aren't Sith.
There might be some things we could glean from the films and Clone Wars, had George not been so sloppy with his writing (e.g., Palpatine -- no Darth whatever -- originally referred, repeatedly, to Luke's lightsaber being a Jedi weapon, didn't use one to try to execute him, and the descriptions George gave for the Emperor was that he was all shriveled up and sickly looking due to being devoured from the inside by the Dark Side... then he forgot all that in the Prequels, and now he's "Darth Sidious", carries a lightsaber, which he uses to fight, and his messed-up face is now due to Force lightning, even though Luke exhibited none of those signs after being hit by far more of said Force lightning...). Good stories, bad execution, by and large. Many contradictions introduced, which we the fans now have to rationalize or try to ignore. But it makes connecting the dots harder than it might otherwise have been.
Also, it hit me the other day... People have griped about George repeating himself all the way back to when Our Heroes had to "blow up another Death Star" all the way back in ROTJ. That, to me, was one of the bigger indications of George needing good writers working with him. He'd lifted the Death Star from the end of that story arc to put in Star Wars, since he had no expectation of ever making enough off Star Wars to warrant a sequel, let alone multiples. So when he got to that point, rather than come up with something new... he just used a bigger Death Star. Then he intentionally paralleled the original Star Wars story in TPM, saying in planning sessions "it's like a poem -- it rhymes". Now I realize what he was trying to say, if he were wise in the ways of music...
Star Wars is a canon. I don't mean that in the way we normally talk about canon on here. I mean it in the musical sense. Go listen to one of the most famous -- Pachelbel's Canon in D -- and you'll hear what I'm talking about. Or much of Vangelis' music. A canon takes a simple theme and then, through repetition, builds on it, making it more elaborate and adding something new each time. The Prequels handled this clumsily, but it's there. Now TFA is the newest repetition of the theme, with the familiar elements being incidental background against which the new is placed. Neither would work without the other. The new needed the context of the old, the old needed the novelty of the new, and they're interwoven.
Yet again, I agree that it couldashoulda been handled a little differently, as far as conveying crucial... need-to-know... information! But I assert even more strongly that if all you saw was the repeated theme from earlier in the saga, you're listening to a rock song and only hearing the bass line. I can't tell you how to watch it, but you're missing a lot that is actually there.
--Jonah