If you're already running wireguard and just want a VPN, there isn't much that you're missing out on except for convenience when it comes to device management and routing, automatic hostname DNS resolution, and also getting access to more advanced features like meshing and failover LAN/subnet sharing without needing to figure out how to do it in bare wireguard.
Honestly though, it's free and makes for a great hassle-free backup VPN that just works. I use wireguard as my primary because it's fully self-hosted, runs at the kernel level instead of within the userspace so it's faster, and is more native than installing third-party solutions; with that said, I still run tailscale on all my servers as well in case I bork something while editing wireguard configs at any point.
For my situation, as a backup/secondary VPN? I haven't really noticed any downsides. It's come in handy couple of times when I messed up the wireguard config on my wireguard server - it's on an oracle cloud VM and it's harder to get a local terminal in there in case you lose SSH/remote access, which I did.