For me, it is the width that is off. The squares can be totally off...I have found the precuts to be okay if you don't need to use every single one. OR if you want 2" strips...you can cut down the 2.5" I buy only brand names from quilt stores, so far not impressed.

I have made the jelly roll log cabin quilt several times. The secret is to square your blocks. The piecing went pretty fast and the results were quite good. Be consistent with the 1/4 inch seam and it should work out okay. Good luck. BTW I have not had many problems with my jelly rolls at all but I usually get the brand name ones.