I am a new PremEx with UA but have been a Super Elite with AC for a long time and I did travel frequently with UA having by bag checked with the Priority tag attached to it, both North America and International. This never works.

One time on an international flight on UA, they put the tag on my bag and my bag got out the last one when I arrived in CDG .

I asked the UA employee standing by who told me (in french): "That is normal, you are not flying business or first class. The tag doesn't mean nothing if you are not flying business or first class."

I had better luck now that I am PremEx with UA.