Had a 100g heavily stocked FOWLR with some hardy corals (toadstools, leathers, etc) for over a year without a sump or skimmer, just a AC500 HOB diy refugium, an Eheim Pro II 2026 & 40w UV sterilizer. No problems with that tank with no sump.
However, now all my sw reef tanks have sumps or above tank refugiums.
Anthony