I have an Athena 11 speed group set, but can get a good deal on the Shimano 9000 chain. Does anyone know of any reports of issues combining the two?
Leonard Zinn has discussed Shimano and Campy 11-speed drivetrain compatibility several times in response to reader questions.
Although Zinn doesn't specifically address chain compatibility, I'm not sure how you could successfully mix drivetrain components if the chains were incompatible.
Read this article, all 11 speed cassette works with all Shimano, SRAM and Campagnolo drivetrain, meaning chain and RD and crankset, so the answer is the Shimano 11 speed chain and Campagnolo 11 speed chain are interchangable.
Given that all the manufacturers spend enormous amounts of time & money on R and D to make sure that all their parts are properly compatible and will behave exactly the way they want them to, it's crazy to mix and match.
My company do work as a contractor for Campag but another part of my company also look after general mechanics training and so we are very familiar with Shimano, SRAM etc at all levels - and if you asked if a Campag chain could be used with Shimano I'd say the same as the other way about - yes, it'll work but it won't work as intended or even as well as the proper item - you will also be binning your warranty and if it all goes wrong and you end up in hospital, don't even bother trying to sue either manufacturer ... the same applies with KMC (here, KMC take up the liability, good luck with that if you ever need to call them to account) and SRAM - designed to work with SRAM of course ...
This is all completely normal and to be expected. The days when you could mix and match with total impunity disappeared some while ago ...