For boxes.dev we use E2B - they had everything we needed at the time. I'd love a standardized API / marketplace, but more just to try out new providers with differentiated features or pricing - we probably wouldn't be switching often / using multiple (unless customers asked us for more options). There are so many sandbox providers popping up!

Our use case is pretty specific though - one persistent machine that you set up your full developer environment on, then ability to very quickly spin off full copies of that machine (filesystem + memory) as separate VMs to run agents on. Firecracker VMs + support for "forking" the box were our key requirements. Another provider that could do this was Modal, but they use gVisor and not Firecracker, which means it's harder for users to run docker inside the box.