Archie has a covered tray - never had any issues with him using it. The reasons I went for covered tray: he doesn't bury his crap but leaves it for maximum smell dispersal, and he likes to scratch the sides of the tray (or the wall if there's no cover) just so I know he's been successful 🙄

Kitty leaves solids uncovered but stands on her hind legs and scratches the wall. Also she often poops over the side. Covered is probably the way forward then.