I like it when actors speak their mind, even if I don't agree
Mark defended the prequels a lot which annoyed me but I admire the guts it took. However on this case I don't think it's that much about hating the sequels, I think it's missing the chance to work with Harrison again
Imagine being on the same set as your lifelong friend and one of the few people who was with you when SW started and they were nobodies, only to not be reconnected on their only chance (since Han got killed). It's gotta be frustrating. We got Han/Leia and Luke/Leia, it was only fair to get Han/Luke. And I'm also bitter about not getting a scene with the three of them
So I get where he's coming from.