Possibly just experience with whos doing the work. This will be my 4th M52 swap. I havent done an M54 (and wouldnt have taken the job on if thats what was going in). Its hard enough navigating LVVTA certification, learning time is not billable time
there is also 20 years of information out there on the M50/M52 swap, which were being put in e30s before the M54 was even launched
Rather keep this thread on topic, but feel free to create another thread about it, or even google, im sure there is plenty out there arguing one way or the other.