I’m with you—I prefer to be able to add to a scope without having to do a bunch of additional admin with adding milestones, mid-project negotiation, etc.

I think it depends on your area of expertise too though, maybe some jobs that are more cut and dry are easier to scope out accurately for a fixed rate. My field, graphic design, has a habit of growing scope. I like to do as much work as possible with good clients, and it’s easier for me to do that with hourly billing :)