I pretty much agree with all of your points here, Stanton. Well stated, amigo!
The only thing is I don't know that I, personally, would be able to choose COMPANEROS over QUIEN SABE or DUCK YOU SUCKER; but, I do understand how one could.
Just a matter of personal preference, once again.
You are right, COMPANEROS does have a great opening scene (as well as a great final scene, in my opinion).
But, like Stanton has said these great scenes that are sprinkled throughout the film can't raise it to the same plane as THE MERCENARY...in my humble estimation.