The improvements in Nautilus are a huge step forward. Better performance, stability and tools. Highly recommend anyone using Ceph on Proxmox to take this upgrade soon.
They are; I particularly like the dynamic pg-num management.
The upgrade instructions on the Proxmox site regarding Ceph are good, too. There is an important ordering of events, and if you stick to that you're fine.
The one single upgrade "problem" I found occurs when you run hyperconverged, i.e. with MDSses and OSDs on the same machines:
At some point in the upgrade, you will have done a rolling restart of all OSDs in the cluster, and you will have run the ceph-volume tool to scan/activate all your OSDs in the new database. Then, the docs say that you should do a rolling reboot of OSD machines to ensure that Ceph correctly starts all OSDs on boot. *DON'T!* At least, not at that point: you will first want to do the MDS upgrade dance, since otherwise that will happen in an uncontrolled manner when doing the rolling reboot. If you run with just one active MDS (plus standby MDSses), as I do, you're fine. If your OSDs and MDSses run on different machines, this is a moot point, too. However, if you run with multiple active MDSses, you may run into trouble there.
Do the rolling reboot after the MDS upgrade in that case.
Edit: the one other thing is that you'll *first* want to remove the monitors' port numbers in ceph.conf, *before* you enable the mgr-v2 protocol.
I'm a Cowboy, and I did the upgrade on a live cluster with all VMs running and with RBD and CephFS traffic going on, and that went fine.