Eeek, I can't imagine what this is like if it scales. What happens to the fire risk when theres 20,000 laptops with aging batteries all sitting together? I hope they take the batteries out, however many laptops use batteries to smooth out power fluctuations. Laptops aren't designed to be servers - peg your laptop CPU and GPU at 100% and see how long it lasts, I've done this before and the answer is about "2 months",…
No fires, no hardware problems. No special cooling other than the mini-split that was in the closet to cool the server rack. They just kept trucking. But modern hardware is much more high strung and I don't doubt you'd have weird failures.
Edit: Back then VMs were how things were done and RAM was seemingly always the bottleneck by a mile, so the cluster did add up to a meaningful amount of extra performance compared to not having it.