Wow, this was fun. I managed to research Machine Tier 7 before it finally crashed. Spent probably 3-4 hours playing this. Was getting slightly slower and slower as I played, so presumably ran into some limit at the end. You can see the minimap of my factory at least from this screenshot after the crash here =).
Definitely ran into quite a few bugs - realized the belts would skip a space pretty early, but that kind of became part of the design constraints of the gameplay at that point =). The factories not being centered on their 2x2 area when they got bigger was a little hard to work with. Worst was them growing in the build palette meant that after tier 4 I had no access to iron bars anymore (no way to place a new furnace as I could find no hotkey for it, and couldn't get to it on the palette anymore), but I managed to work around it.
I didn't like being forced to use the new, larger assemblers/sellers/etc. I almost got hard-locked when I reached a new tech tier, and the seller was now super expensive compared to the previous tier, more expensive than I could afford, and I didn't have any seller placed down anywhere near my science pack production chain :D. Managed to route things back to an old one without doing too much damage and earn enough for a new, high-tier seller.
The little units that produce higher-tier components without inputs were fantastic. Price felt just about right (they're the only thing, besides that one seller, that I found myself saving up to buy), and they're very satisfying compared to "okay, now, let me double the size of this boring part of my factory" and let me focus on the new, exciting part. I'm curious what other factory games you've played might have that as a concept (because I'd probably enjoy playing them), or if that was something novel?
Individual pixel art was nice, everything felt recognizable and pretty unique, even all of the components - I found myself able to just use the icons to recognize what I need to craft rather then needing to double-check the names, which was nice. If only the tooltip tips on the assemblers also used those icons so I could tell at a glance which one I was lacking =). Overall composition felt a little inconsistent, with things being scaled different sizes as they grow bugging me a bit.
Maybe interestingly, I never realized, until very late, that you could cross belts - since the underground was unlocked initially to solve that problem, I didn't even think to try until I did it accidentally (and at the point I figured it out, I was too scared of belts mixing due to bugs, I mostly avoided it :D).
Overall, really awesome work, I've enjoyed this one more than any other game in the jam by far =).