Personally, I don't think the method is problematic at its core because of the law of diminishing marginal returns. Perhaps I'm not going to get a 100% perfect shirt form mytailor, but I can get pretty damn close and to the casual observer, the difference will not be too evident. So, unless you have some kind of crazy deformity a la the Hunchback of Notre Dame, then I don't really see a reason why you can't get a reasonably well fitting shirt from mytailor. Even if it has minor imperfections, you can still get a shirt that is substantially better than a RTW shirt.
Having said that, I have a bunch of shirts from mytailor and I think they're pretty good. Sure my CEGO shirts are much nicer and fit better, but, when I like to play around with details just to see what will happen, I use mytailor. Also, I like waiting for stuff in the mail.