My off-site backup is a thinkpad x230 with a 1 TB HDD. It's currently at my friends house, and I access it with tailscale. 7 eur/month to colocate this in a datacenter with stable (and fast) Internet + power seems like a pretty good deal.
I can understand some of the concerns with user-provided hardware. Maybe a better model, would be for CoLaptop to offer hardware themselves. This would allow them to standardize on a few models, which opens up many possible improvements such as central DC power, power efficient BIOS settings, enclosures with cooling ducts, etc. They can still follow the "old laptop as a server" model by buying off-lease laptops from the corporate world.