Why isn't sith sorcery used more frequently in combat?

From an in-universe standpoint.

There aren't that many Sith that can use it unassisted, but even most of those who can don't frequently actually employ it (e.g. Sidious, Valkorion). But then there are others who *do* use it a lot, like Andeddu and Zannah.

Maybe those who actually use sorcery are disproportionately good at it relative to their skill at conventional techniques?

You answered your own question.

Yeah what you said. Look at King Ommin perfect example of a Sith Sorceror. He was very strong and had hax being able to an armies will but against Ulic he basically got one shotted although Ulic had to bulldoze thru his magicks.