See, I was already getting confused with the sheer amount of material out there. I followed the Jedi Academy series for a while, and tried the NJO, but just couldn't get in to it. Then the Vong, the Legacy and then I gave up.Pre-TPM, everything from the Thrawn Trilogy onwards was considered "basically canon" simply because it was felt there was never going to be any more films, so these new adventures was all we had. When TPM came out things changed. There were films again, and the thing that no one really wrote about directly, the Clone Wars, was a possible thing now. Afther that set of movies there had been some major changes and the EU adapted as best it could while moving forwards with the Heroes of Yavin (who had not changed much in the PT) and with new stuff around the new films. Then TCW started and threw a part of the EU books around it into the Great Heap. But the Heroes of Yavin were still untouched by this. Only when it was finally decided there would be sequel movies did the now long line of novels about the Heroes of Yavin post- Endor become "well that didn't happen in this version of events". Maybe cause we never really thought a sequel would be made anymore.
Does it change that the novels exist? No. Does it take away from the enjoyment they caused? No. Will it be difficult to get people who have not read then the read them? I don't know. There is a large potential for confusion now with a large number of novels verse both TCW and TFA. Not so much Rebels as that was an area that wasn't all that explored, and when it was it was via a known character. So far nothing is really off with Rebels verses what we've know before outside of some details that were covered mostly in roleplaying games rather than the older novels.
About the only things we don't know if they happened or not and probably won't because I doubt the canon will shift back to it again, is all the Ewoks materials (the cartoon and the two TV movies). Wicket's adventures on Endor are easily isolated from the rest of the universe and could have happened or not without disturbing the rest of the canon.
I might try them eventually, and I have several Legends books because I like them. So, I have no issue with Legends and Disney canon sitting side by side.