How are the conditions in horse slaughterhouses?

As a top horse breeder, I send several of my unwanted and overbred horses to slaughterhouses in Mexico. However, I have actually never been to any of these slaughterhouses, as I just give them to a person who drops them off there (I live in Texas near the border). Are the conditions sanitary? I mean I know those places aren't the cleanest,and it's not the greatest experience for horses, but are they at least treating my horses humanely. Serious answers by horse experts only please