Specifically on avatars made from the "Nebbia" base model (from booth) - and potentially others, undiscovered - there seems to be an issue with the elbow IK that doesn't allow the wearer to go cross-armed.

First picture is on legacy IK, this is pretty much exactly how my arms are in real life (and how I think it looks natural), this is how it looks with beta IK on about 20 other avatars I tested. On this one specifically, however, I can't cross my arms, as shown in pictures 2 and 3 (made with beta IK).

I believe the armature is set up normally, I compared with a working avatar and could not see major differences.

I suspect this is an issue with my avatar, more than with IK, but it is technically a regression (for me anyway). Please let me know what more information you need, I'm happy to help debug this issue!