Incus supports both qemu and lxc

You can also start QEMU/KVM powered VMs with Incus, I assume that's also possible with IncusOS?

It's a lot more than that. Clustering, storage drivers, networking, etc makes up a whole virtual machine manager. It never says it's a hypervisor, it's a VMM as outlined on it's github: "Powerful system container and virtual machine manager"

I have switched to incus and it's really great. It's lightweight, has a working terraform provider, easy-to-use cli, pre-built images (LXC and VM) of major distros, runs on any distro (on proxmox, you're stuck with debian), clustering is nice, supports bunch of storage drivers (dir, btrfs, ceph, zfs), simple web UI and active community. The project leader is also very active and helpful while in proxmox, it's a little unresponsive. You can even install `incus-base` package which only contains LXC specific components for only running LXC containers.

I have noticed incus has better security configs by default. For instance, all pre-built images come with secureboot enabled and there are ACLs which are easy to configure for fine-grained network rules. The only downside I feel like is lack of something like PBS

Profiles are really great. It's like cloud-init on steroids

Incus is more comparable to LXD than proxmox. IncusOS is different though.

LXD containers also are unprivileged by default.