>cost of resources across customers.

IMHO, this is all a solution chasing a problem. A server from Hetzner costs the equivalent of 30 minutes of my time a month. A VM on a shared machine can cost even less. There's just not cost there to save in order to justify the security risks and performance ghosts. It only makes sense for extremely bursty loads or background processing where latency isn't important. But those are pretty atypical scenarios and usually buying for max load is still cheaper than the engineering time to set up the system.