MI machinery doesn't accept power from TR, but TR machinery does accept power from MI generator

This is working as intended, as TR machines could quickly screw up progression

This makes sense. But what about battery boxes and MFEs? They don't produce power.

This makes sense. But what about battery boxes and MFEs? They don't produce power.

You may use them as a medium to transmit power to MI machines, thus unusable. Tho you can probably make a MI battery or something

Yes, this is intended. Only MI generators and cables can convert EU into TR's Energy, and it can't be done the other way around.