The basic problem is that heat transfer limits guarantee that fusion reactors will be big and expensive (meaning: power density an order of magnitude worse than a fission reactor core, and probably worse than that.)

The core of a PWR fission reactor that one can build today has a power density of 100 MW per cubic meter. The power density of ITER (dividing the gross fusion power by the volume inside the cryostat) is 0.05 MW per cubice meter, 2000x worse. Other fusion designs aren't quite as ludicrous, but will still be much worse than fission.

The problem is that heat in the PWR has to get out of fuel rods that are 1 cm in diameter. The heat in a fusion reactor has to get out of a plasma vessel that is meters in diameter. The ratio of surface area/volume is orders of magnitude higher for the fission reactor.