The legal work I did was for an expert witness case as a subject matter expert (not a lawyer). There is no way we could have/would have agreed to a fixed fee up front unless it was for a really outrageous amount. (Well, it ended up being a pretty outrageous amount anyway but there's no way we could have reasonably scoped the work.)

Most of the work we did in general was for specifically scoped deliverables and we actually tried very hard not to do day rates, much less hourly rates, but this was one case it really wasn't avoidable--and, of course, was the way the white shoe law firm operated.