I don't know about CX specifically, but I have indeed had multiple check in agents telling me how tired they get of people asking for free upgrades, and in some cases how much they dislike it. There have even been a couple of completely separate cases when I was upgraded at the check in counter and when thanking the agents was told in no uncertain term that it was because I had not asked anything.

I think the one exception is the US where people seem expected to ask for those things anyway, but in many other regions this just comes across as rather awkward and mildly embarrassing.

On the original question, I can confirm that many of the airlines which use FFP as formal criterion tend to prioritise their own status members over partner members of even a much higher status. BA and AF, by contrast, tend to use a fairly opaque system which is as much of a "secret recipe" as some famous liqueurs. Many even suspect that a higher status is not necessarily a bonus in some cases.