It has problems with VoLTE where the loudness setting of the handheld is ignored. For me, the VoLTE connection would also stall often while roaming - not sure is this is related to the Pro1 or to my carrier.
However, I am in the comfortable position that German carriers will support GSM (ie. non-VoLTE) calls for years to come, so I just disabled VoLTE. Haven't had any call issues since.
Regarding @sequestris saying the Pro1 is stuck on Android 9: This is true for the vendor-supplied ROM. There are very mature LineageOS ports from 16.0 (Android 9) all the way up to 20.0 (Android 13) i