I admit, I'm super-skeptical about such products, and your inability to locate scientific literature in support of them is exactly why. They don't likely do any harm and may in fact do some good, but I'm not at all convinced that such products justify their expense from the perspective of longevity of pond function in whole.
Those products tend to be proprietary concoctions of facultative bacteria, some substance (enzymes or micronutrients) to promote their growth, or some combination of the above. Because they aren’t labeled as any kind of pesticide with the associated licensing requirements, many (perhaps most) makers don’t publish their ingredients, keep those ingredients as trade secrets. Those "beneficial" bacteria do indeed consume muck/organic material. However, those bacteria need oxygenated conditions to be effective. Without oxygen, they’re not efficient; with oxygen, some related bacterial assemblage will be there and active anyhow.
I search this literature periodically; I still can find no professional, peer-reviewed literature that supports or refutes the notion that these products substantially enhance water quality or slow the pond-aging process in whole. Thus, I'm skeptical regarding whether or not adding a jug of formula or handful of spikes or pellets to a volume of water the size of a pond or small lake can make a lake-scale difference. However, some formulae may have useful localized effects (e.g., aggressive application around a dock or swimming area). Also, aggressive short-term treatment may “jump start” a depauperate bacterial assemblage in the short term. Again, without peer-reviewed literature, it’s all speculative.
Creating healthy habitat for beneficial bacteria by keeping dissolved oxygen circulating along the sediment throughout deep water (as with diffuser aeration) seems to me to have much greater potential to increase bacterial populations and effectiveness. It’s usually recommended that bacteria treatments be made in conjunction with aeration; in such an application, it’s difficult to know how much recovery can be attributed to aeration alone. Thus, on pond scale, bottom aeration seems to me a better initial investment.
Please let me know if you have additional questions.