(An EU that covered movie, games, books, etc, thus maybe unfit for the OT)
When Disney took over LucasFilms, the EU was (famously) reseted. To the dismay of many fans (including me, who have bought pretty much every Star Wars books of the Old Eu, except Karen Traviss ones). When you think about it, it was a sort of necessity-the EU was simply not adaptable into movies. (Think about how a very simple plot point like ''The Republic funds covertly the Resistance'' was poorly explained in SW VII-how you could have crammed the Vong War, the second galactic civil war, the Emperor clones, the peace treaty with the Empire...in one movie ?) But when the EU went off rails ?
IMO, as someone who have read pretty much all the books...
A)A rather improbable succession of events for a strictly canon perspective. The Old EU followed a strict canon (IE, everything happened, not ''what you read is something that could have happened'', like for instance one of the dozens of iterations of Batman) Kylo Ren EU counterpart, Jacen Solo, had the following life before becoming Darth Caedus
1)When he is two or three years old, the Emperor burst into his nursery and try to steal the body of his younger brother Anakin (that sounds stupid, but that was in Dark Empire, so it goes with the setting)
2)A few months later, he is attacked by a Sith Lord, Exar Kun, and end up defending the comatose body of Luke Skywalker (it sounds stupider, but that was in Jedi Academy , so it goes with the setting)
3)He is kidnapped for the first time a few years later, and is brainwashed as an Imperial agent
4)While on vacation, he is then kidnapped by his dad racist cousin (and incompetent Slobodan Milosevic expy. I mean he is incompetent a being the tinpot racist tyrant of an hellhole.)
5)Presumably, he is kidnapped or victim of a couple of murder plot in comics (I know for sure of one in Crimson Empire, but did not read all comics)
6)Now a teenager, he goes to Jedi school and have a couple of facepalm inducing adventures (including at least two kidnappings and brainwashing plots, being rejected by Jedi girls (after slicing off their arms...) as well as fighting drugs. I'm not kidding. It sound extremely stupid, but it was in Young Jedi Knights, which make the Phantom Menace looks like Citizen Kane)
7)The superinvaders from beyond the galaxy invade, and Jacen is immediately captured on the very first planet they invade. He finds love with a designated hot scientist shackled to the same gnarly tentacled gimmick. Jacen is captured and shackled to the said tentacle device a couple of times (The scientist will eventually dump him to go study the torture tentacles and the owners of the said tentacles)
8)Jacen tactical advice leads to the vaporisation of half the Republic fleet at Fondor
9)Jacen realize that he mights not be the best Jedi around and give up on the Force (finally). But then because of this his mother his tortured (with tentacles) in front of him
10)In one of the most facepalm moments ever in StarWars, the Yuutzan Vong design a bioweapon to hunt Jedi (who are around 100 at this time) Luke, showing stellar tactical accumen decide to send a fifth of his entire order against the factory of the said bioweapon, the one specifically made to hunt Jedi. All the teenagers and one veteran. After seeing his brother die and half of the strike team, Jacen is captured
11)Jacen is captured and brainwashed. He is let, Theron Grevjoy style, escape to be recaptured and retortured. (Three times). He make friends with a Yuutzan Vong terraforming device, that torture him (There was really a problem with torture in the Vong arc...) ...
After all those experiences, Uncle Luke let Jacen walk the galaxy all alone, to meet various Sith psychos. Result ? (While highly sarcastic, this is a very summarized list of what happen to Jacen before he turns dark. In addition to the mockery about being easily captured and tortured, the accumulation of kidnapping and brainwashing plots ends up making the whole thing more or less credible)
(the other possible expy of Kylo Ren, Ben Skywalker, have it considerably easier. While he is often captured and tortured, he attracts rather the attention of pretty Sith ladies who certainly want to convert him to the Dark side, but by bedding him)
B) In the Del Rey era of books publishing, there was an attempt to integrate everything, which lead to rather farcical plotlines, as it was usually not the good elements that got from books to books.
For instance, in the god awful ''Jedi Academy'' trilogy, we are introduced to a cluster of black holes, the Maw, that house a super top secret weapon ressearch facility (one that keep churning godawful superweapons). It's really a whole new kind of stupid, as this facility can't communicate with the outside and have a military leader unwilling to contact the outside after 10 years of silence (the Empire can apparently lose a top secret weapon facility that worked on the Death Star and the four capital ships guarding it). It was at best a servicable concept. But the Maw keep getting used
1)For housing Jedi kids during the Vong war (it would not be such a stupid concept if, judging by the map, the Maw was deep inside not only occupied Vong terrirtory, but inside territory that never was really controlled by the Republic)
2)And, just happen to be the place where superadvanced aliens built a superadvanced prison of blackholes to keep one gnarly superevil force entity captive (thanks Luke for providing her force morsels to eat during her captivity)