Thanks for the nudge, Blinko.
I'd left the uc_migs module in -dev release pending either (1) my own testing on a live site or (2) someone else doing same and reporting that it worked. To answer your question, then, "lack of feedback". But I should have left an issue open saying "plz post feedback if it works for you", although realistically ... wheels only squeak when they need attention, right?
A couple of sites appear to be using it and haven't reported issues, so I'll push it to stable now. Feedback and (good) bug reports always welcome!
http://drupal.org/project/uc_migs for download.