If a typical overflow plastic tank fails, no big deal. The issue here is the same as we first had with NC generation Miata, part of the Fiat 124's plastic tank is actually a pressurized part of the cooling system. When the plastic tank degrades a little hairline crack can develop under the cap and the cooling system leaks all pressure through the cracked plastic and the motor overheats fast because no pressure equals much lower boiling point. In the case of the NC generation Miatas, which was the first car on which we had this problem, we eventually made an all metal replacement tank for them because NC with failed motors from overheating started to fill up our lot waiting on new engines. So far we have seen a surprising number of coolant leaking Fiat 124, not just from the plastic tank, but several from the plastic tank with hairline cracks just like we first had with the NC miata. Thus, we decided this time to not wait until our lot is full of Fiat 124 waiting on new motors. See the Miata forum threads on expansion tank failures at Miata.net, you can read for days about it, here is one of many of those threads.
We were thankful when the ND generation Miata arrived and we noted that Mazda is back to what we all know and love, a standard plastic overflow tank that is NOT a pressurized part of the cooling system. Mazda had learned the lesson the hard way. We were thus a little bummed when our Fiat 124 arrived a year later on same ND platform and we noticed Fiat's car on the same platform is again using a pressurized plastic tank that we know will fail, and potentially allow the car to quickly overheat.
We also have Fiat 124 customers racing these cars for autocross and track and they need more effective cooling systems than even a totally healthy stock Fiat 124 has brand new, particularly if they upgrade to a stuffed turbo. Our strategy for those customers is our upgraded BIGGER ALL ALUMINUM Radiator below matched with a higher pressure cap for the expansion tank. Higher pressure makes the system more effective, raises the boiling point, etc.. Because we didn't want to just stick a higher pressure cap on the OEM plastic expansion tank that we know is already suspect and vulnerable to early failure with hairline cracks, we decided we needed to offer a durable aluminum coolant expansion tank as the best match for the new all aluminum radiator.