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.
ok, but what if you need to complete a task and cannot assign or find employees in a reasonable amount of time? would you then consider contracting the task?
Of course, short term contractors are ideal for overflow work. My point is that I try not to rely on part time contractors over the long term because their availability can change.
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.