I did SFO-MEX-EZE on MX several times, and it was just fine. Weather you need to go through immigration or not depends on what terminal you land and which one you leave from. This two things seem to vary quite a bit. But immigration has always been very simple for me in MEX (with an Argentinian passport, and a Mexican wife).

Other options to consider might include TACA: SFO-SAL-LIM-EZE, Aeromexico, also SFO-MEX-EZE (they have only started SFO-MEX recently, and their new lounge on the new terminal in MEX is fantastic!), or, just for fun, LH: SFO-FRA-EZE (where you get to fly LH's longest non-stop flight!), NZ: SFO-AKL and AR or LA AKL-EZE, KL: SFO-AMS-EZE, etc., etc.