Both the Bluenote and the UP-4 are unipivots. Speaking theoretically, many unipivots seem to lack the stability to fully control lower compliance cartridges like Shelters. As Ozzy apparently discovered, they tend to limit you to higher compliance cartridges.
I don't have Ozzy's experience with such arms, advice like his led me directly to gimbal-based designs to mate with my Shelter. Even the inherently stable arm that I chose benefitted further from stability-enhancing mods and adjustments. I can state with confidence that maximizing stability in the arm is crucial to fully enjoying a Shelter, or probably any other lower compliance cartridge.
I don't have Ozzy's experience with such arms, advice like his led me directly to gimbal-based designs to mate with my Shelter. Even the inherently stable arm that I chose benefitted further from stability-enhancing mods and adjustments. I can state with confidence that maximizing stability in the arm is crucial to fully enjoying a Shelter, or probably any other lower compliance cartridge.