If you want small and plan to keep it simple and have a low bio load, they are just fine. I think they do well if you keep up with doing regular water changes.
Personally, I like having a sump and even a refugium, even on my 40g cube. And not just a small one. Mine is a 40g split about 30g for the fuge and just 10g for the sump that has the skimmer and return pump both in the same chamber. I'm not big on water changes and I like that the sump and refugium help me keep the system clean in a more natural way. I only have 5 fish but the tank is wall to wall zoas and Rock Flower Anemones with just a few lps and sps corals and a few other inverts.