Pellets do not mimic a wild gliders diet nor could a prepackaged diet replicate the fresh gums, pollen, sap, and insects found in their natural diet.
Pellets are manufactured in large facilities that are not up to human grade standards, anything can be in those ominous chunks.
All pellets claim to be fully balanced diets on their packaging even though a captive gliders diet has not received enough attention in scientific settings to know for sure what a glider needs, therefore you know right away they are already lying to you.
As J&Z'sMum pointed out, Sugar gliders break their food down in a very special way, a way that requires many liquids for them to properly extract nutrients otherwise they may become malnourished. They also have a ceacum for breaking down gums and vegetation that are often deemed "indigestible", many of the pellets do not contain things like pollens, gums, nectars, or the like but are instead a generic pelleted food meant for rodents that was repackaged with a Sugar Gliders face on the front.
Most pellets are extremely high in Iron, I don't think it is just a coincidence that one of the leading causes of death in gliders in kidney/liver failure, this usually happens because of an abundance of toxins like excess vitamins.
PS. I am well aware of how popular cat and dog foods lack basic nutrition, they are often far removed from the animals natural diet and that is why health problems in pets has become so common. These products are also produced in a way that is dangerous almost as if a living creature wasn't going to eat it. These foods barely keep domesticated animals alive, how in the world would basically the same food keep an exotic animal alive?