Of course, some of the issues are related to the individual's tolerance of the various products. I think it is prudent to include some protein in the mix on longer races. You can find protein, electrolyte and caloric totals for these products, as well as their glycemic indexes (and glycemic loads...something that is starting to be a hot topic) and make an educated guess at what will work together well and what won't.

But, I'm not convinced the glycemic index values in relation to blood sugar levels and insulin levels hold true in long races, i.e., some companies espouse the idea that their relatively lower glycemic value products shouldn't be mixed with higher-glycemic value products in order to avoid insulin-associated "bonking". Others take a different view, saying the insulin-associated bonking doesn't work that way in an exercising athlete, and simpler carbohydrates are the way to go for the entire event. I mixed the two concepts and did fine.

I used G-push 2 and 3 mixed together, a few Hammergel Endurolytes capsules, and took 3 Powergels on a half ironman bike-split. Oh, and plain water. I used the provided Optima and water on the run course. I was mostly concerned with total calories and electrolytes rather than with whether or not the calories were from lower or higher glycemic sources.

This was in mid-90 degree temperatures that had over 200 of the 600 participants treated in the medical tent during/after the race, and over 40 people hospitalized locally. I personally know of additional people that were hospitalized several hours after leaving the race in locations other than the local facilities.

Although some of these products specifically recommend not to mix other things with them, I did fine with this concoction in what I'd consider severe conditions. I consider myself somewhat lucky to have correctly estimated my nutrition/hydration/electrolyte balance/pacing in order to finish in good shape. On top of that, I even had a very respectible placing in my first 1/2 distance attempt. And a good deal of the credit for this success belongs to this forum, where information is available and dispensed in such a way that you don't have to act like a newbie in a race distance, even if you are one.

You do have to practice with actual race-day strategies to be sure you have something that will work, but, this is difficult to do. Compared to your training efforts, the course conditions change, the effort may change, water bottles are dropped, etc. The best you can do is be knowledgeable about how to adjust on the fly, so you can maximize your potential to finish the race at your best pace. Sometimes you just need to be lucky, but the more you know and prepare, the more often you'll be lucky.

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)