The u flag will use the wav file directly from the source. It is similar to importing the sound file directly into your DAW. You can't stretch the wav by doing so. This also means that with the u flag, the Me flag is actually redundant i.e it does nothing with the u flag. As to why some people prefer using tn_fnds (or WARP), it is because it is the only way to make Maiko sound good with a longer note in UTAU. It might also be harder to stretch or extend the length of the note in the DAW and you might not want to hassle yourself with it since the looping feature already exists with tn_fnds itself within UTAU.
You can actually replicate the u flag with any resampler by adding $direct=true to the note settings. However the main point of using moresampler is due to moresampler's superior crossfading capabilities. This can only be achieved by using moresampler as both the wavtool and the resampler.
I think you would have seen this video tutorial :
I haven't tried it, but in theory, there is actually a way to lengthen the notes with the u flag; by setting new voice configurations for the vowel part of a syllable, then crossfading it multiple times in succession. But as I said it's a hassle and just looping it with tn_fnds or WARP would be easier.
