I have a biopellet reactor onmy 120g. Been running it for years. I had one on my previous tank, a 150g as well. I have always like it, it has kept me from needing GFO or carbon, and helps keep phosphates in check.
I just this week took it offline just to try it without for awhile to see what differences that I notice. My fear is that the biopellets are keeping the water to clean and stripped down, robbing my coral of nutrients. If I have a ultra low nutrient system, I would need to feed coral heavily and add things like amino acids. I only do both occasionally. My growth has seemed stagnant for awhile, and my thoughts are the water is too stripped of what my coral needs. I have seen a burst of coral growth this week, since I took it offline, but I also boosted my Ca reactor to add more calcium too. So it could be one or the other or both. I'll see how things go over the next couple of weeks.
By the way, I use the NextReef SMR1 reactor and Warner Marine EcoBak pellets.