I’ve been using Biopellets for approximately 6 years now if not more, and found it to be the most convenient and effective way to manage nitrates as well as boosting phosphate consumption.
Unlike other methods, it allows me to manage nitrates very accurately by doing it in the opposite direction - it’ll plummet to zero which I’ll than increase and hold where I want by dosing Sodium Nitrate, which in turn also increases its effectiveness in phosphate reduction.
Another point I like with using Biopellets over the traditional Carbon dosing is the minimal to no PH impact which you often see when dosing Vodka/Viniger.
One thing that needs to be pointed out though is that it really needs to be perfectly dialed in to work properly and prevent the issues others mentioned above, which is mainly controlled by having the right amount of pellets and flow, as well as directing the output of the reactor right into the skimmers intake.
Once dialed in - your skimate will get thick and dark, with almost like oil consistency.
The only cons I see with it is the long break in period and the short amount of time the media can survive without flow, as well as the time required to learn how to perfectly dial it in.