I kind of get what you are saying but it might really just be dependent on the quality of the image the bootleggers get. There are some daki's I have where the image quality isn't near on par as some of the others, they also happen to not have nearly as many high-res good quality scans or pictures available either. There's also the fact that different prints on different fabrics will come out differently. I have seen various different prints on different materials and some are really crisp and clear, while others have nowhere near the detail.
There's also the longterm quality of the daki as well. Over time no matter what you do, if you make use of it, wash it, etc... It will slowly fade, depending on the print quality and material has pretty resounding effect on the matter. If you really are that distraught by it perhaps get a very high quality picture of the daki and get one custom done so that you know the quality of the image source.
A lot of time bootleggers end up just taking custom orders they've done and start reprinting them from whatever picture was sent in so it's probably an issue of image quality that they come from.
You are right in that the printers are the same so the only difference in quality would be from the base image.
Honestly if I had the budget and could more readily find another dakimakura that I wanted a couple copies of I'd test this out getting an official version of a daki that I know the material of and then getting another bootleg of same material and submitting the image myself. Unfortunately all the covers I want either don't have known scan or artwork to submit, are of an unknown textile, or very rare and or expensive.
But as the other poster said don't let one bad experience ruin it for you, and they are significantly cheaper, and infinitely easier to replace.