What, if any, might be legitimate halachik concerns with uncertified Nature Valley granola bars? Their "Crunchy" line of products seems to be certified as OU pareve(?) However, many of their other lines of products don't appear to be certified at all.
One example is their (dairy) Sweet and Salty Nut Almond bars, which lists as its ingredients:
Almonds, Corn Syrup, Whole Grain Oats, Sugar, Rice Flour, Palm Kernel Oil, Whole Grain Wheat, Vegetable Glycerin, Honey Roasted Almond Butter (almonds, honey, maltodextrin, palm oil, mixed tocopherols), Whey, Fructose, Canola Oil, Salt, Soy Lecithin, Corn Starch, Barley Malt Extract, Baking Soda, Natural Flavor, Vitamin E (mixed tocopherols) Added to Retain Freshness.
Likewise, their (nondairy) Fruit & Nut Trailmix bars list:
Whole Grain Oats, Corn Syrup, Almonds, Raisins, Sugar, Roasted Peanuts, Rice Flour, Vegetable Glycerin, Cranberries, Canola Oil, Fructose, Corn Starch, Soy Lecithin, Salt, Barley Malt Extract, Baking Soda, Natural Flavor, Mixed Tocopherols Added to Retain Freshness.
It seems highly implausible that the natural flavor might include, e.g., nonkosher animal products, especially considering that the same is listed in their OU-certified products. Is there enough basis to assume that they are indeed kosher, at least bediavad (e.g. after purchase; or once in one's mouth)?