Myself I never had the "opportunity?" to work flat rate but one thing I heard from a guy I knew who worked in a bodyshop where the work was flat rate is if the service writer or person assigning people to jobs has a buddy or two it can be rough. Buddies get all the new rust free cars and the other guy gets the old rusty farm truck with caked on horse sh*t.
As a young Auto/Truck tecnician, I worked over a decade Flat-Rate. The above statement by kshansen, is absolutely true. Buddies got the easy money work, some got really screwed, and the middle ground you made a decent living. I saw it from both ends. The way they hire and fire service managers and service writers, it was like a lottery.
In saying all of that, I did really well; of course I was doing a lot of work the others could not do.
Remember, I was much younger. Flat-rate can be hard on your health, mental and physical. You will see slow times and busy times; your bills will come whether you are in a slow time or busy time. So you have to manage your money.
Not every body can be a Flat-rate technician, but a good one is like Gold.