Mousses can fail (look at Dakar this year) but it's very rare at the ISDE. I doubt Mexico will be as hot as Greece was (35-40°C everyday - 90-100°F) and we never had issues with the mousses. Use decent amounts of lube (not too much if you're not using rimlocks) and it's a non-issue. The one place mousses tend to fail is prolonged high speed road riding. This wasn't an issue in Greece, and we never had issues in Chile either where the speeds were quite high at times (one of our trophy riders was up around 150km/h on the highway sections).
Don't even think of using tubliss. FIM tires aren't very durable, so I guarantee you'll get a flat using the tubliss system. Tireballs are great in a rear tire, but suck in the front tire in rough conditions. They also don't like heat. I ride with my head over the front fender, so the front end takes a pounding. Combined with the heat, a little over half way through day 1 ALL of my tireballs were flat in my front tire. Unfortunately, I wasn't prepared so I had to ride the rest of the day on a flat front tire, pounding through rocks. I had to slow way down to preserve the wheel (can't change hubs) so I lost about 23 minutes on day 1, and was physically spent from wrestling a bike with a front flat for over 100km.
If you want reliability, use mousses. For the ISDE you'll need 4 of them - 2 front and 2 rear. Get brand new ones to start with, properly sized for the tires and don't bother with rimlocks.
For non-tubes in adverse conditions, mousses are generally the most durable, followed by tire balls then tubliss.