as far as mass productin goes, the U1 is the most difficult. for a start there was only ever 1 of them. if you do the LMS garrett then you get 30 and with slight alterations you can have all 33 (the first 3 had fixed bunkers, the other 30 had the rotary bunker).
Also its extrtemly difficult to fit a mech in the lner garrett. i know this sounds strange, having a massive loco with such little space, but try it sometime! i have done the LMS garrett and the biggest thing i could fit in was 2 DS10's which are frankly crap.
the LNER version is much worse because of all the air under the watertank. people will expect it to pull like crazy, but motor size is a real issue.
Peter