I just completed upgrading my Call Manager lab to 12.5 and thats on ESXI 5.5, didnt have any issues with the upgrade process, although there was one additional COP file I needed to upload via SFTP for Unity Connection v11.5 upgrade to v12.5, this COP file addresses a defect CSCvi08763.
I haven't discovered any other issues yet but it is a lab and as for production environment I wouldn't consider using anything below ESXI 6.5 for v12.5, triple check recommended design guides etc