I'm not sure if you guys have kept track of this, but it seems as though ever since Celebration, Mark Hamill has been getting more.....vocal....about his thoughts and experiences with both TFA and Episode 8.
There are actually a lot of interviews on Youtube about this right now, but this one seems to encapsulate everything:
Now obviously, Mark has to do a bit of promotional dancing here. I'm sure that there are a lot of agreements in place that would prevent him from saying too much about it, but what he has said so far has been....not good. Reading between the lines, it's downright worrying. When you have Luke Skywalker's actor saying he "fundamentally disagrees" with the direction that they're taking his character in, my alarm bells go off.
What could this mean? We won't know until December, but if I were to guess based on Hamill's remarks as well as Daisy's remarks about "being disappointed by your heroes", i'd say that we will see the character of Luke portrayed negatively, possibly gone crazy or a danger to those around him, and likely dead by the end of the movie (also based on Mark's looser lips this time around and rumors that he's gaining weight again).
Whatever the case may be, I think it's almost 100% guaranteed that the hero we know from the OT and who we've been invested in for 38 years will either have his legacy tarnished or his character vilified in some way, over and above his failure of Yoda's hope that he restore the Jedi and his cowardice in running away from the problems of the galaxy and his friends and family (which is already bad enough).
I was not a fan of TFA at all, so that may color my views on this, but i'm feeling the small hope that I had for Episode 8 diminishing.
I think we might be reading a bit too much into this. I really don't hear anything terribly worrying in this interview. He had one idea in his head, and was surprised at what was in the screenplay, and while he "fundamentally disagreed," he also said he's never been happier about being wrong.
“I’d seen Looper and Brother’s Bloom and Brick. His films are all so diverse and all so ambitious in their own way. In this experience he has rocketed it to the top of my all-time favorite directors, believe me.”
I think this is a case of you looking for reasons to worry vs. there being actual warning signs about the movie. For what it's worth, I trust Rian Johnson's storytelling instincts over Mark Hamill's (even though I love him). I don’t think Rian would write something that would fundamentally betray the character of Luke Skywalker. I trust that he would be bold and challenge the character (and the audience), which might catch Mark off guard.
Mark also believed that Luke should have witnessed the death of Han Solo, even though there was no practical way or story reason for it to happen. He just wanted a moment to exist, irrespective of how that moment would occur. Kind of like how I might want there to be a Yoda reaction to Alderaan being destroyed in ANH, even though it would serve no story purpose, and would destroy his reveal in ESB.
I've heard Hamill's comments on this, and to be honest, he's just voicing what many fans thought I.e. that Han Solo's death had very little emotional resonance. As others have said, it sounds like he was at first unconvinced about Luke in TLJ, but Johnson turned him around.