I think some are afraid of how much of an investment they have put into the game. Seeing it come to fruition (or not) means that either some people have WAY overinvested themselves in time and/or money, or they see a game coming that they would not have backed if they had known the extent of the changes.
For me...I don't know. I have a lot invested in the game and I am not liking some of the direction the game has taken. I liked the whole "choices have meaning" thing and that death will matter. I thought that maybe it would be a game where people will really think about their choices and decide that being an asshat wasn't the best approach. (yeah, that was stupid of me thinking that people would actually think and act like they would on the street on the internet) But we also get a game where half of the point seems to be shooting others. So, if we engage in combat and lose...well, it sucks to be you because there are consequences to dying, but if we don't engage in combat, we lose half the game. Yes, I fully realize the "get gud" remark will come.
Yet, no matter where we go and what we do to "get gud" half of the people in the game will be below average when it comes to combat. That's just the law of averages and nothing will change it. If it gets to a point where people feel they can't progress because of these combat issues, and they can't do the other part of the game because they feel combat will be forced on them, then what is there for them in the game? Something is going to have to change in the game or CR will lose the PvE crowd due to the stunts/actions/heroic piracy/whatever you want to call it. PvP will be a small part of the game according to CR...well, that's changed too.
So, these parties have realized that the game is not what they envisioned, so they are blaming everyone but themselves.
"Mining behind a paywall, so you can't test" (admittedly, some backed when all you needed was supposed to be a mining laser), when in fact there are lots of ways to test and contribute to the game.
Another cry is "the game has changed and it's the backer's fault for staying silent on earlier changes" Game is expected to change. Some ways are good, other not from my viewpoint, but change was expected.
Finally the whole "CR has still not delivered what he promised back in 2016 [3.0 with good networking] We still can't play the game and we are still stuck in Stanton." Playing the game comes after beta. Right now we are in alpha and priorities changed. Keeping things small and getting it right in the small areas is a good way to find the things that are wrong and fix them once and for all. Then expand on the good foundation that you have laid.
I feel bad for them. Yes, some purchased/funded/pre-ordered with the expectation it would be out quickly. That's not what happened and yes, there may be a case for some of the original backers to get their money back in they so choose...and CR should do so quickly and quietly with some sort of "ok, we refunded you with the provision that you "go away"" and don't badmouth SC in the future. After all, what you really don't want is to upset the majority of your backers and it seems that by delaying and pushing back, they may have let a few rotten apples start to spoil their barrel full of good ones.