Most are hourly. Done that.
Some are on commission. I would be thinking service plumbers mainly for that.
I have also worked piece work. Done that.
Normally salary is for supervisors. Done that.
I loved piece work. My first plumbing job was new construction, a shop with a staff of 150. After I got my forty hours in by Thursday, I was allowed to work piece work on Fridays. Funny, allowed to work piece work instead of getting time and a half. Hmmmmm.
With a helper and an apprentice, we were roughing in five three bath homes a week. Granted these were slab basements that the helper could dig the groundwork for, and the floor joists were open so I place my pipes through them without drilling. Hillis Homes.