Using the E-Assist when the trailer is empty could certainly cause problems but when fully loaded most of the energy would be used moving the trailer, Now if you don't pedal and are using the trailer to move everything then I can see problems happening... As to where the danger cut off is 350W, 500W, 800W..? I think a lot would depend on how you use the assist... Helping to get up the hill with the loader trailer or also pushing the bike to get up the hill because you don't want to pedal...