Any EnderIO generator don't output power to Energy Cubes, even with a EnderIO cable.
Universal Cables don't touch EnderIO machines, and don't transfer power to them.
Tried every possible side configuration.

Thanks, I forgot to mention. Using 228 of Mekanism, and the latest of EnderIO.
I'll install CoFH Core to test, maybe it's something API related.

I'm not seeing this issue... what version of Mek are you using? I have (ultimate) energy cables attached to all my EnderIO machines, and just tried it with a EIO Stirling generator, and all connect just fine.

Can't be API related, EnderIO is shipping the RF API files it needs to work.

Confirmed, installing CoFH Core does makes everything behave as it should. Are Mekanism and EnderIO using a different version of the API by chance?

Currently "intended" - Mekanism only interacts with RF if CoFHCore is installed. This may change in the near future.