Why are simple sugars bad?
Simple sugars (glucose, sucrose, fructose, dextrose, etc.) need to be mixed in concentrations no higher than 6-8% in order to achieve an acceptable absortion osmolar value of body fluids (280-303 mOsm) and be digested with any efficiency. That's it. The problem is that a 6-8% solution is a pretty weak mix and will only yield about 100 or so calories an hour, which is inadequate for maintaining optimal energy production. Some athletes realize that and try to resolve the problem by making a double or triple strength batch of their simple sugar product. Unfortunately, that solution is now far too concentrated, it's much higher than 6-8% and, unless more water is consumed or added to the mix (at which point the athlete might very well be flirting with over hydration) that concentrated simple sugar solution will not pass the gastric channels. Energy production is compromised and stomach distress is sure to follow.
The same problem occurs when an athlete combines a simple sugar fuel with a complex carbohydrate fuel. The beauty of complex carbs is that they will match body fluid osmolality, not at a 6-8% solution, but a more concentrated 15-18% solution. Even at this seemingly too-high concentration complex carbohydrates (such as maltodextrins/glucose polymers) will empty the stomach at the same efficient rate as normal body fluids and provide substantially more calories (up to three times more) than simple sugar mixtures will. However, when simple sugars and complex carbs are consumed together or near each other, it increases the solution concentration beyond what either source can be efficiently digested at. In other words, when you consume simple sugars and complex carbohydrates together or within close proximity of each other you negate the efficient digestibility of either source. Once again, energy production will be compromised and a variety of stomach issues are likely to occur.