Is it true there have been some quality control issues with ROTF Mixmaster such as misassembled parts?
Depends on how you look at it. Is Mixmaster exactly the same as in the instructions? No, he is not. Neither is Starscream, though. Toys change through development. Does Mixmaster transform just fine as-delivered and do all of his bits and pieces fit where they're supposed to? Yes, he does and they do.
Some toys do have the front cab section assembled "the wrong way". One of the little bar pieces is on backwards compared to the instructions and both types are out there. Mine's wrong, but it's not a major issue, he can still be transformed and is still a bundle of fun.
I've only ever seen and heard of the "misassembled" version. But as has been said, and in my own personal experience, its not an issue at all, everything still transforms and fits together just fine. Only QC issue I've had is that I had to take off his knee pads because one was really loose and I've no way to fix it/can't be bothered to. I think one of the joints that connects his wheels to his arms is loose, not sure. Other than that he's perfect.