What didn't you like about NJO Jacen? (Personally, he was always a character I could take or leave; either just another young Jedi character or a kinda lame Kylo Ren-like character before TFA came and made one that was actually good.)
I didn't like the philosophy Vergere was spouting that he took to. He also became extremely arrogant, thinking his stupid philosophy was right and just generally acting like an ass. You're right about him kind of being a proto-Kylo, but not done well. It got worse later, Darth Caedus was terrible even if I ignore the fact that he killed my favorite SW character in a pointless shock death for stupid reasons, and it all started with NJO. Before that I thought he was ok, a bit generic and not as interesting as Jaina or Anakin but not bad. but, really, once he met Vergere he rapidly went from ok if a bit bland to irritaiting to infuriating.