I switched from i3 to Niri a couple weeks ago, and I've been super happy with it.
Niri feels like it lines up more naturally with the way I tend to use windows and workspaces. I'm working on one project per workspace, opening an occasional ephermeral terminal window or web browser to the right when I need to reference something or run a quick command. My other windows in the workspace aren't altered by these new ones, no reflow happens, and then I can close it when done.
My only problem with Niri is that now I really want an Ultrawide monitor.
I have a 49” monitor and tried Niri for a while. I had some issues regarding Wayland, I believe because of Nvidia and stopped using it. I could probably solve them, but I have been using X since years and I don’t feel switching right now.
Anyway, for that short amount of time I liked most of it on my ultrawide monitor. Except when you open just one application, it stays on the left-most part naturally and it honestly sucks to look at. I have no idea if I could modify the settings to launch apps with an offset and eventually occupy the complete screen estate. I‘m planning to build a new AMD machine and will try Wayland compositors again for a longer period of time. Niri is my first candidate.
You can. You can also center them by default, or toggle centering via keyboard shortcut, etc. it’s pretty flexible.