I fly (well used to fly until they axed the more convenient DFW-MEX-DFW flight) AM quite often in the last ~14 months. Mostly international (DFW-MEX) and a few domestic flights. I was first FB Platinum and now FB Gold. I got upgraded twice out of the roughly 20 flights - once the flight got cancelled 20 min later and I was rebooked to the next day () and once I got upgraded on a very cheap last minute flight I booked (Basic Economy).

Speaking of Basic Economy, at least through web chat AM will not upgrade you to AM+ seats ahead if you don't book Main Cabin. They will always refer to the check-in counter. I still think that's fair as that is what I paid for.

Typically I am not checking a bag but when I do I had to argue with the AM check-in staff at DFW that I get it for free due to my STE+. Could be just the outstation. Checking in at MEX, even in the SkyP section, can be a lengthy pain though, as they are not the fastest in processing passengers.

Lounge access in T2 (AM + DL) is never a problem for international flights. The lounge is not too bad.

One last thing: when they cancelled all MEX-DFW afternoon and DFW-MEX evening flights (same plane turning around) as of September 1, 2019 they did not hesitate to rebook me on Delta connections, even allowing to change the departure date by one day. I found out they cancelled the flight to use the E190 they used on the MEX-DFW-MEX route as a replacement for the grounded B737 MAX on another route.