So many factors go into the strength of a seat and the structural integrity it imparts to a hull that it is impossible to claim that a rotomolded seat is always better than some other type of material used as the seat. These factors include the shape of the seat, the quality of the materials used in its production, the number and shape of the points of contact with the hull, the nature and location of the attachment points to the hull, etc., etc. as simply example, think of the different support capabilities of a flat strip of thin plastic (push on it lengthwise and it bends incredibly easily) compared to the same thickness of plastic with a simple 90 degree bend (like an angle iron)... very difficult to bend lengthwise. I am quite sure that a thermoplastic or composite seat can be designed to be as strong and structurally sound as any rotomolded seat.
Further, how many documented injuries, drownings or other bad things,if any, are there because of the use of a thermoplastic versus rotomolded seat? In fact, how many, if any, such events can be attributed to the failure of seat made of any material instead of other factors? I don't know the answer to these questions, but my guess is that the numbers are very, very low.
If you don't like a boat, just say so, but don't try argue a boat is structurally unsound without any basis for doing so.