I have seen a few boat sterns get stress cracks from hitting a pothole with all that weight bouncing off the back for hours.
Also - A heavy outboard motor on a bracket is NOT designed to take the weight at 55mph on a trailer. No matter how much you have enforced the transom - I would get some way to at least get the weight OFF the motor mount during trailering. We remove mine and place in pickup truck bed.
1977 North American Spirit 23