I've been a "professional" longarm quilter for nearly eight yrs. In that time I've only returned one quilt to a first time quilter that had mitered something like eight borders. I felt really bad but I knew that quilt was beyond my capability to "work my magic" (that's a comment quilters often tell me when they leave a quilt with me).
Flaws that make my job difficult:
Ruffled borders, too thick seams that stub my hopping foot up, poor pressing, batting or backings cut too small, quilts that are not square, open seams. I know there are a lot of longarm quilters that go the extra mile and take off those ruffled borders and fix them but I personally don't do that, if needed I'll send it back with the client to remedy and then have her bring it back for quilting. I do fix any open seams that I find, as it needs to be done before the quilting.
I do know that being a longarm quilter has made me a better quilter piecer. In fact, I told that to a client of mine that just purchased a longarm.
I have never felt that I'm the quilt police, my job is to complete the process for my clients to the very best of my abilty. I love my job and I've met the nicest people all over my area and beyond. Quilters and quilting is cool!!!