There was disconnect between Kelly Services and who was actually contracting me (NIH). Their yearly evaluations had no basis on day-to-day reality, and they played small role in my tasks or task management
When there was government shutdown for 2 weeks, contractors never got backpay, only actual government employee
Mediocre benefits -- health plan was awful, though this was before current regulations so perhaps they're better coverage now, though most likely meagerly subsidized
Finally, there was very little career opportunity. 1 out of the 20 or so contractors that passed through in my 2 years there got promoted, and that was after he worked as contractor for 5+ years. Not great return on time investment.
Very little investment in career growth. Took what I could from the NIH where I was contracting, but no classes were reimbursed, no educational conferences, etc.