I wanted to provide some early good, bad, and ugly feedback as someone who is enthralled with this theme, and found the demo to be enticing. I haven't sunk anywhere near the amount of time I'd normally forfeit to do this, but I felt enticed to do so given how open your team appears. If anything, take this as a first response interpretation of the game where I've played it both solo and with friends.
The good:
The aesthetic is very welcomed, and a return to older concepts which seem to be generally forgotten. It reminds me a lot of Stranger's Wrath, with sprinkles of early Insomniac Games (Ratchet and Clank the most immediate). It also has moments which feel kindred to older 2000s games like Armed and Dangerous with the absurdity and gung ho nature. Given there are references to Serious Sam minions, it's obvious to me that the developers have a solid palate to pull from. This makes me more excited for the growth than anything else. The little trinkets of joy found in the game - squeaky shoes, magical pigs, and so forth, is pleasing and actually gives me confidence that there is potential for novelty.
The bosses are a great addition, and are much welcomed. Co-op games can struggle with solid boss design especially without MMO-lite mechanics in play. While I would like for there to be more novelty to how they're handled - similar to something like Remnant - as is is still great.
The abilities are fun to use. The nuclear minions are a fun chaotic element. I like the horse - and the Witcher reference. I'd love for more unique weapons to be explored.