Idk if any of this information will be beneficial, but from what I am seeing so far...
If you open an utau ust in notepad, and an utsu ust in notepad, they look similar, but read different things. On the left is the utau version of the ust and on the right is the utsu version.
Few differences I noticed, first in the [#Version] section is obviously the difference in the versions. Then in [#Setting] I noticed the ust opened in utau does not have the time signature, but has the track number and a place for cache'd files. It also lists the resamplers I prefer to use (Which is a personalized setting I have for my editor..)
The biggest difference I noticed is with the notes themselves, which is the [#0000] etc ones.
I took a closer look at the area between #0002 and #0003, which would be the space/rest between the third and fourth note, and this is what I saw.
Basically, they look the same. So I went back and edited the ust in utau to give it the rest/space it's supposed to have like it does in utsu and this is what I saw.
The one on the left is the utau ust without the rest/space, and the one on the right is the one with the rest/space. It reduced note #0002's length down to 480, rather than 960 like it is in the first utau one and the utsu one. From what I am seeing, duration=/=length. Length is how long the note is, and duration is how long the note is actual audible. In utau, we don't have duration in our usts. So I see this posing a problem with the cross compatibility of utsu usts and utau usts.
Again, I have no idea how useful any of this information is, I'm not a developer and I know nothing about stuff like this. I'm just trying to help to the best of my abilities.