Carbon rims are plainly more delicate than alloy rims. How delicate and to what degree varies. Each carbon manufacturer/rim model uses different quality materials, each with its own chemical properties, reacting to heat and lateral load differently. Two carbon rims can have drastically different reactions to using PedalCell. We stray on the conservative side and don't recommend carbon rims for this reason.
We've seen some riders try PedalCell with carbon setups with varying success. But we don't track them diligently and cannot advise on how a particular carbon rim will hold up. If you mount on carbon, you should be aware of the risk.