I have bought several kits. On a couple, I have been shorted fabric (17 by 22 FQ!!) or some of the fabric was so thin you could not use it. Ticks me off for the price you pay.
I mostly buy them when I see a quilt done in a colorway that I would have picked myself. My pet peeve is when the kit is in a package and once you open it, it is not exactly as the sample. If they are going to substitute, I can do that!
Another thing that irks me about kits is when they dont include the binding. Backing I can understand but if you finish the thing and then have to hunt for fabric to match for the binding, it's a hassle. I would just rather pay a few bucks more and have it all.