It's not my area, but I suspect that the heat extraction rate for a fusion reactor must be several orders higher than current fission reactors if we are to run them continuously. Fission reactions can be actively controlled by using reaction moderating material (control rods), which means we can tune the reactor activity to match the amount of heat extraction available.

Fusion plasma must be sustained at a few million Kelvin or it shuts down again, and I'm not sure how finely we can control the operating temperature without either overheating the reactor or shutting down the plasma. I think it will take a lot more than dumb engineering to accomplish this.