People base their knowledge on experience. I am a member of IMSS and have found their emergency care to be very good. They have delivered babies, done heart bypass, taken care of the aged and sick. All of the above are people I know or have known. I went to the small clinic in our little town for scratched lense. I was in and out in 15 minutes, complete with examination, cleaning and medication.
IMSS also offers PREVEMIMSS which includes a yearly physical, all the blood tests you could ask for and dental exam, cleaning which includes all non-cosmetic dental work (they do prep work such as posts, fillings, temp crowns).
Now, IMSS also farms out some surgeries and rehabilitation. Less drain on the system and the service providers are top notch. We have used both in the last two years.
I am in the vespertino, or afternoon group. I can go anytime and make an appointment to see a doctor. If I just want a basic test such as blood pressure checkup, I can go anytime, put my carnet (booklet) in the box and wait to be called. I can tell by the number of people in line if I want to wait or not.
For the small amount I pay, I think I receive quite a bit. Transplants are also done as well as dialysis, premie care, all in specialized hospitals. I've been to many of the specialized hospitals and they are in very good condition.
I'm not just talking about Monterrey. Don't base all your assumptions on one or two visits to a small rural clinic as it doesn't represent the whole picture.