Niri demo video actually looks kinda cool, could be nice to use on a laptop when there's no access to multiple external monitors, so that you could just pop a log/tool/whatever window to the side without fulling swapping workspaces xmonad/i3/hyprland/etc style.

But with 2+ screens available I'd think at least for me the usefulness would diminish, even if you'd have per monitor scrolling

I'm using niri with two screens at work and it's been very nice. I don't open windows on the side as you suggest but I believe that can be done with custom bindings and/or window rules.