The willful ignorance people have of the reality in front of them will never cease to amaze. The demos and videos that they release are what they have deemed acceptable to show to the public as an example of their work. Not only is it an acceptable time to give your thoughts and share some concerns on what you've played and seen, but it is the best time to do so. If you're raising a concern that a developer knows will be taken care of by the time their finished product is released, then there is no problem there. If your concern is shared by others, and becomes prevalent enough that it ends up getting back to the dev, they can easily address those concerns if they know it will not be an issue. Or, if they think that a sufficient portion of their audience does not like the direction their game is going, they can opt to change course and give them what they're looking for. Or not. Totally up to them. As long as they're not misleading people about it, there's no issue and they can do whatever they like.
This is how feedback works. It is not harmful, it is a very normal and healthy system. Not giving any feedback on any of the demos or videos you've seen throughout development, and only giving feedback upon the full release is significantly less useful. At that point, the game is already made. You are highly unlikely to see any significant changes or improvements, especially with a small project like this.
If you don't care about any of this and you're just waiting for the final product to judge for yourself, perfectly fine. Nothing wrong with that either. Do that. But, don't be coming in here saying that everyone else needs to be quiet about what they've seen so far. That they shouldn't be putting any level of critical thought into it; to put the blinders on, and ear plugs in, and pray for the best. If people want to talk about the development of the game, they absolutely can.