Not sure if this is the intended behaviour, but the size of the internal tanks in the basic mixers (both the RU ones as well as the electrical ones) are much larger than in vanilla GT6.

This makes automation with backstuffing quite hard, as well as being fairly unbalanced, since a single block bronze mixer can hold 8 million litres in one tank (vs. 192k litres in vanilla GT6) which is about the same as a small tantalum hafnium carbide tank.