I know that Firecracker does not let you bind mount volumes, but QEMU does. So, we changed to QEMU from Firecracker. If you run the workloads in Kubernetes, you just have to change a single value in a yaml file to change the runtime.
I would be scared to let unknown persons use QEMU that bind mounts volumes as that is a huge security risk. Firecracker, I think, was designed from the start to run un-sanitized workloads, hence, no bind mounting.