I have never been a fan of psychedelic horror. As I define it, psychedelic horror is any film designed to use fragmented character development and storytelling in a psychological thriller shell in an attempt to elicit a fear response. The reason I don't much care for these films is that the fear factor rarely works because you are left too confused and bewildered by what is happening to feel any real dread. It is near impossible to pull your audience into fragmented structure to any effect.
Some films have been able to do it effectively, but even they have incidental moments of levity and confusion that breaks the tension. One such example would be Sphere. Most of the movie establishes itself in a grounded environment, which helps, but the climax is laughable if you aren't in the moment, as our protagonists sit in a functioning submersible yelling at each other that they can't escape. Jacob's Ladder and many David Lynch films can be seen as successes that I fairly enjoy, but as a whole, especially when the proper writing and acting aren't in place, these movies fall flat by failing to define which way is up.
In The Last Winter, we are subjected to similar failed results. The majority of the film is senseless, and perhaps the worst part is that it is meant to be. Our characters refuse to make sense of their actions, even though the direct correllation of said actions to any outside stimuli isn't properly established at any key moments. Mysteries are near impossible to solve when the entire cast is going insane. It basically leads to a third act in which you realize that nothing has been accomplished and everyone is essentially walking in proverbial circles. Yes, we are given "answers" to the situation that is happening, but they are really guesses from mad men, which equate to irritating verbal garbage when considered in the context of storytelling.
The ending helps provide some clarity as to the extent of what took place at the arctic base, but after spending nearly 100% of the film's runtime having accomplished practically nothing from a storytelling perspective, it is little consolation to shed some light in the final 30 seconds. The film goes back and forth between some type of viral outbreak, to gas inhalation, to spectral manipulation/possession/murder/suicide. Showing monstrous, transculent beasts "attacking" people that may or may not be insane gives the audience nothing, and frankly is just a hair above The Happening of being laughable. Invisible threats can be a powerful catalyst for horror, but when done wrong it can turn a film into an MST3K wet dream.
It's unfortunate, because Ron Perlman is a great time in most films he is in, and to see him in something I could barely sit through makes me disappointed in myself that I couldn't see the script for what he apparently thought it was worth, though let's be honest, Perlman doesn't have a history of being the pickiest actor.
Perhaps I'm being too harsh, or perhaps I've missed the point of the film beyond the "nature will eventually fight back against the greed of humanity" trope. Maybe the story fell into place perfectly for everyone else. Maybe the ambiguous plot dynamics being left to the imagination was exactly what you were hoping for. In that case, my ratings below could be subjectively divisive from your own, and maybe you shouldn't take the ramblings of an undead sociopath too seriously when it comes to films with ambivalent peril.