Being the middleman in my experience for the national services is never quite profitable. They're all different so it doesn't hurt to check it out.
To address your concerns (again it depends on the outfit) but on time payments usually happen, however they make you jump through hoops aren't typically are 60-90 days.
Yeah middleman, you aren't going to make nearly as much, the contractor takes the bulk.
You're a sub, you have no control. You do what the specs require. Most have the facility's manager check your work and sign off on it. It it doesn't clear payment is delayed.
If you need quick money just starting out they are good. You'll want a lawyer to go over the contract cause they aren't in your favor.