When niri runs applications it will now put them into transient systemd scopes. One concrete benefit is that when an application uses too much RAM and systemd-oomd kills it, niri won't go down alongside the app, so the rest of your session will stay intact.
Does Gnome do this? I've certainly had my entire session crash when a certain LSP server used up all of my memory. I appreciate this feature!
I think it's time for me to try Niri as my main WM. The main thing I want to figure out is getting XWayland going so my Wine games will work. I know there is info on this in the Niri docs, so I'll start there.
Edit: The key to getting the games working is gamescope! It runs a nested X session. Lutris does not work without X, but Bottles does and it has a handy gamescope checkbox in the bottle settings.