Yah sometimes they do, but sometimes they don't work either, because the battery is dead.
Personally I get tired of the snobbish attitude of many "experts" that assume that operators in Mexico are all sloppy, that you shouldn't trust their gear, that you get better training up North etc.
Last week for instance, we had a diver show up for diving with a dead computer practically begging us to get a battery. This happens often enough, believe me. When we recommend divers with their own gear to come a bit early to set up so we can make sure everything is working right, if they say, no it's ok, I just had it serviced, that rings the alarm bell. It is all the more likely that it will have a problem. We see it all, from regs that free flow because the intermediate pressure is improperly set to leaks of all sorts, ,often from hoses that are not tightened. I've taken free flowing regs apart when the customer actually had the bill with him for a full service and the seats were worn and had not been replaced, oh yes, it was clean inside but so what, it didn't work!! In one case it was a Poseidon reg and the guy had spent well over $1000 getting a new second stage and a full service. Well it was the wrong second stage and didn't match the first stage he had and also required a different pressure setting than his other second stage and therefore free flowed, the pressure was set all wrong. We ended up getting a new second stage for him and setting it up properly, he won't need to service that baby now for about 10 years at least.
The Riviera Maya, Cancun and Cozumel have around 400 dive shops in total, sure some of them are crap, so shop ahead, use local knowledge, go with small to medium sized accredited shops. There are other, perhaps more important aspects than the gear, like professionalism of your guides and captains. Send them an email and judge them by their responses to your questions. If you still have doubts then just buy your own gear then and pay the $100 that most airlines now charge for sports gear bags.
If you are properly trained, you should be able to deal with any equipment failures in any case, so try not to worry and concentrate on having fun.
Thorndog