Speaking as an employer, availability is important. If you're only working 15-20 hours a week for me, you've surely got other commitments that are occupying your mind and time. I've also seen many people doing part-time contracting take on more than they can handle, either moonlighting part-time from their full-time job or taking on too many simultaneous gigs. I've done this myself as a contractor, so I understand why it's happening, but as the employer I don't want to be on the receiving end.

I still hire part-time contractors for things, but for anything that's time sensitive or availability is a concern, I prefer to have a full-time employee.

Speaking as an engineer that sometimes wears a hiring manager hat. Full-time long term is typically preferred. On a complex product in a complex domain there can be a long learning curve. It’s very valuable to have people that stay for multiple years and were there when certain choices were made.

People have a different learning and onboarding speed, but it generally takes months for an engineering hire to “break even” and be able to pick up most tickets on their own. So we generally prefer people that stay longer.

That being said, there is also value to bringing in people with different backgrounds, experiences & perspectives. So sometimes freelancers or consultants can be good choice.