Ask HN: How is software consulting different than working on an internal team?

I have been a web engineer for a while. In my early days I did some work at agencies, mostly on ad work. I didn't care for the fact that everything was rushed out, tracking billable hours and not being able to iterate and grow software over time.

I've spent the better part of the last decade working on-staff building internal projects, and have been frustrated by the bureaucratic slowness and how behind on technologies we are due to brownfield projects. Those seem to be mostly the exact opposite problems I had early on, although I absolutely do not miss tracking billable hours.

I am considering switching employers and wanted to know how life at a developers-for-hire company is. I would especially like to hear it contrasted with being on a software team at a medium-large corporation.