End of last week upgraded a mix of 6, 7 and 8 Proxmox VE servers to 9. Restoring containers/VMs from backups when necessary. Had issues with NFS shares over port forwarding I use for backups on Proxmox 9 but have just worked around using CIFS. Proxmox setup of ZFS Raid1 on new metal was pleasantly straightforward.
TBH, biggest headache was a couple of containers running old centos 8 and Ubuntu 18.04. The former I've given up on and left the centos container running on Proxmox 6. I'll migrate its services to Ubuntu later. The Ubuntu 18.04 container I incrementally upgraded to 24.04 and had a few hours sorting out PHP errors (new PHZp version of course) from the Apache server (internal only) it was running. No rush to rollout Ubuntu 26.04
Honestly I can't believe how straightforward and glitch free the upgrades were. Proxmox is easily the most user friendly hypervisor out there.