I finally gave up on BioPellets after running them for a couple of years in a few different tanks. I was very reluctant to give them up. For the most part, they were an easy form of carbon dosing and my nitrates and phosphates were very low: 0.0 NO3 and 0.03 PO4. Adding them to an existing tank meant dealing with a cyno bloom. This was not an issue for my tanks, but could be a big problem for sensitive stony corals. This can be mitigated by starting out with a small amount and adding to it over time.
That aside, the reason I gave up was bacterial blooms (white cloudy water). It only happened a few times, but once I got a bloom, the biopellets seemed to feed it and it would not go away without a lot of aggressive water changes and finally a UV filter. If nitrates again become an issue, I may add one back, but if I get a bloom, I will quickly turn on the UV filter and remove the biopellets.