Don't know, but the Vetus ones sure are. My Vetus plastic diesel tank is 11 years old with no signs of deterioration at all. I also like the fact that it is translucent so you can see the fuel level through it.
The tops of the plastimo tanks leak at the slightest provocation,like when the filler is located above the top of the thank and the intake hose is full.The weight of the fuel in the tube is enough to overcome the O ring in the lid.They're cheap and strong though.
I agree properly made polyethylene fuel tanks are a blessing to the yachtsman. One word of caution, you have to be able to fit one of the manufacturer's stock tanks into your space, hopefully without sacrificing too much capacity. Having one custom made will up the price 5-10 times, because of the cost of the mold.