I initially noticed this situation with the 510 field, which supports u but not 3 (then drops u in bibframe2marc...)
By looking at the specs to determine which subfields are and are not supported, I looked at control subfields (https://www.oclc.org/bibformats/en/controlsubfields.html) to see where they were not supported. Here's a list that I think you can easily add support by just using the existing templates from the control subfield stylesheet:
$2
v034/s2
v047/s2
v377/s2
v380/s2
v382/s2
v506/s2
v520/s2
v856/s2
$3
v050/s3
v380/s3
v510/s3
v524/s3
v544/s3
v563/s3
v758/s3
$u (URI)
v370/su
Literal - the following are ones I think could be supported only if you change the conversion to not use a literal value:
I initially noticed this situation with the 510 field, which supports u but not 3 (then drops u in bibframe2marc...)
By looking at the specs to determine which subfields are and are not supported, I looked at control subfields (https://www.oclc.org/bibformats/en/controlsubfields.html) to see where they were not supported. Here's a list that I think you can easily add support by just using the existing templates from the control subfield stylesheet:
$2
$3
$u (URI)
Literal - the following are ones I think could be supported only if you change the conversion to not use a literal value: