cost - you pay in your own CPU cycles/bandwidth.

reliability - obviously, the startup would have to earn the reputation for reliability, but there's nothing inherently stopping it.

etc - AWS is a beast, relatively speaking, and this offers a lot of smaller PCs for a short amount of time. I can't really think of a reason where that would be needed for computing, but as network relays it would be very useful. You could create your own custom Tor and deploy it on demand.