One gotcha I found with facets, with a workound fix included:
My music is tagged such that compilation CDs are tagged with "Various" in the album artist field, with the individual artist in the regular artist field. In the case of collaborations, there are multiple artist fields set, one per artist. This works out quite well in facets, I can use
$if3(%<album artist>%,%<artist>%,'('Unknown')')
to format the artist column so I can have albums show up under both artists in the case of multiple artists. However, if you want to apply swapprefix formatting to those fields, you can get some (imo) unintended consequences. If I use
$if3($meta_branch_swapprefix(album artist),$meta_branch_swapprefix(artist),'('Unknown')')
prefixes are swapped as expected, but any entry that doesn't need to be swapped no longer shows in the listing. I would have expected the fallback to be to reproduce the field without any prefix swapping.
For now, my workaround is to use the following formatting for the artist column
$if3($if($greater($meta_num(album artist),1),$meta_branch_swapprefix(album artist),),$swapprefix($meta(album artist)),$if($greater($meta_num(artist),1),$meta_branch_swapprefix(artist),),$swapprefix($meta(artist)),'('Unknown')')
but I'm wondering if anyone else has any ideas on how to do this better.