Re Rydon, let me take a wild stab in the dark guess. No facts, just speculation:
Don't see how you could backpedal without the motor backpeddlin' along with you. I mean, wouldn't that strip a gear trying to spin the motor? Therefore you prolly got 500W of coaster braking assist. More than you may actually need. And once it locks up, all current in the motor with no back EMF to restrict it, or movement to keep things cool. Like going straight uphill in high gear, backwards, only worse.
Maybe why they think unsafe? Firmware should be smarter, but perhaps it isn't. Not even a thermal sensor unless you add one. 350W limit might just be a substitute for lazy. Smaller current limit for reverse should solve it. If it already had that feature, backpedal Watt limit would be meaningless and never challenged.
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I didn't get the coaster version, and have never tried it. Traded my 3i coaster away for a CSA 6speed automatic, still in the mail somewhere theoretically. Right now with no rear hub, no spokes, no decided front or rear brake plan whatsoever. Can't exactly ride it like this. A little worried my TSDZ2's chainring sprag will drag more than the CSA derailleur can pull, excess chain could pile up at the top. But until it all arrives (if ever) I can't even make a plan to deal with all the unknown unknowns. Autoshifting with 500W should be interessent, if'n it doesn't fold from such abuse right off the bat. As long as front freewheel drags less than the rear, it should at least downshift to 1st without having to think about it, and that might help.
Did this test fit with chopsticks though. Half-baked scheme still needs to do something about inside face of motor housing. Its all clamped against the top edge of the American bracket and force is not well distributed. If I really wanted to fix this correctly, Bikewow makes an adaptor with a ring that spans a BB30 shaped depression on the inside face. I may just fill the problem area flat with JBWeld, so the entire circumference of my bracket might support the inside face. No sense having it crack in two, as someone in this thread showed us earlier.