I can confirm there’s a hard limit of 40 “formula” functions in Ranking. They do take up resources so we set conservative limits. It’s the first time I think someone has hit the limit.
The use-case is interesting. For example if two factors rank 60/60 then they average 60; the same as if one ranked 100 and the other 20. Doing a product does favor 60/60 quite a bit more than 100/20.
Maybe we can offer an additional option at the Composite node level which currently offers: Higher better, Lower better, and Summation. We could add Product as well (which is then normalized back to a score from 100-0). Would be the same thing you are doing with FRank but with considerable less compute effort.