The lack of mudguard wouldn't be an issue, almost no mountain bikers fit them to their bikes. There may well be enough space in the frame without a bottle cage to attach it given it's incompatible with full sus bikes anyway, so that may not actually be an issue, could always shove it in a backpack afterall.

The biggest issue to me is the inability to put the seat all the way down given that's quite important, and obviously this won't work at all with a dropper post (which are fairly common nowadays), honestly actual ebikes are quite popular in the mountain biking scene at the minute and they seem like a significantly better fit to me.

I wonder how much torque the motor could put out though without putting a lot of wear on your (probably fairly soft material on mtb) tyres or slipping on knobby, possibly dusty/muddy, tyres; that itself might kill it off for offroad climbing.