How are the conditions in horse slaughterhouses (need information for school report)?

As the son of a top horse breeder, we send several of our unwanted 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, but are they at least treating my horses humanely. Serious answers by horse experts only please