Lots of red dots and prism sights are being used at the matches I attend. Money and Preferences are the usual deciding factors. If by open reflex sight you mean open emitter pistol slide sized optics like the venom I have not seen them used other than as a secondary side mounted optic.
What do you mean by Scoped Red Dot?
If you mean Lower Powered Variable Optic, I have not seen it done. I’m set up for it with a FFP 1-4x Illuminated LPVO that drops down to a red dot @ 1x and a horse shoe dot @ 4x but have yet to run this set up in a course of fire nor have I seen anyone run anything similar in person.
With the distances most shoot I think a red dot with some hold over will do. I like the LPVO setup the best personally but haven’t given it enough time to decide which is fastest for the game being played. You won’t go wrong with a closed emitter red dot.