If for example a new major PVE version will be released 3 months before the next major PBS version, you won't be able to upgrade your PVE for 3 months. So not great if you want to be up-to-date.
You also can not first upgrade PVE, check if the PVE upgrade won't break anything and only then, after verifying this, upgrade your PBS. You will have to upgrade both at the same time. If there is then a problem (hardware not compatible anymore or whatever) you won't be able to run your VMs and you also won't be able to access your backups anymore to be able to wipe PVE for installing the old working version again.
2x PVE (with no PBS) + 1x PBS as qdevice (with no PVE) in a cluster + 1 PBS offsite (with or without PVE but not clustered) would be fine.
That would be the minimal setup for HA and ransonware protected PBS with offsite backups.