...and with no way to run ancillary scripts and cronjobs e.g. log management, backups, security tools... Nor ssh into the VM to inspect it and run debuggers or monitoring and metric agents.
Essentially you are giving away all the practical advantages of modern OSes. There's a reason why the usual famous tech companies don't use unikernels, generally speaking.
As for memory management, in the Solo5/ukvm case the amount of memory you grant to the guest (monitor process) is essentially the maximum you expect the guest to need, ever. So it's similar in terms of overhead to many usermode allocators which just mmap() a huge block of memory at start and then hand out to the application from that.
FV guests can similarly have a balloon driver, that doesn't mean the interface isn't awkward.