I’ve got an SDC flat which has never been the slightest trouble running at full weight. It was built to my own spec and is very strong indeed. Weight wise? on an FM Volvo 3 axle unit, around 16.5t with a full set of full height log pins and a rear steer axle.
I have a Dennison also, which is a good trailer too, probably better finished than the SDC.
I would think though if you want a true one off, the smaller manufacturers like Hingleys may be the way to go. The trailers always have a “proper” look to them.