I use a sump on a seahorse tank. No real downside imo.
Besides the advantages to running a sump, there are some seahorse specific advantages. Microbubbles from a skimmer can disipate before reaching the display. Potentially dangerous equipment can be located in the sump, like heaters, powerheads, etc.
I did have a small horse get her tail stuck in the overflow teeth once. Overflows can be covered with mesh if you have small horses.
One other thought ... if fry are born in the display, they will end up in the sump.
