There are things proxmox definitely can't do, but chances are even if you know what they are, they probably still don't apply to your workflows.

Most things are a tradeoff between extensibility and convenience. The next layer down is what I do, Debian with containerd + qemu-kvm +custom containers/vms, automated by hand in a bunch of bash functions. I found proxmox's upgrade process to be a little on the scuffed side and I didn't like the way that it handled domain timeouts. It seemed kind of inexcusable how long it would take to shut down sometimes, which is a real problem in a power event with a UPS. I also didn't like that updates to proxmox core would clobber a lot of things under the hood you might configure by hand.

The main thing is just to think about what you want to do with it, and whether you value the learning that comes with working under the hood at various tiers. My setup before this was proxmox 6.0, and I arguably was doing just as much on that before as I am now. All I really have to show for going a level deeper is a better understanding of how things actually function and a skillset to apply at work. I will say though, my backups are a lot smaller now that I'm only backing up scripts, dockerfiles, and specific persistent data. Knowing exactly how everything works lets you be a lot more agile with backup and recovery confidence.

Good point, I like the ability to choose between VMs and containers. If I had TrueNAS in one VM and Nextcloud in another, how would you link Nextcloud to TrueNAS? SMB share?