That's true, you do end up with some windows hidden or partially visible. Niri is still tiling, though, so with proper management you can avoid making too much use of the infinite strip (though that would defeat the purpose of niri).

This seems like a good place to note the "center window" keybinding for windows that don't fit well in the screen (e.g., 2/3 wide pane next to 2/3 wide pane, or 1/3 pane on the right end of the stack next to a full-screen pane).

Vastly preferable to having to look at the edge of the screen.