The biggest problem with the ET is that it is very complex, with internal components that mean it has to be hand assembled and bonded together. The failures are mostly due to failures in that bonding process, and the fact that early ET's did not have enough internal bracing in critical areas. I am sure that if they could do it over, they would. It is just too complex, and it relied too much on the integrity of that bonding agent. Even if they had made the pieces with an interlocking joint that was then bonded, it would have been better. Instead, they are bonding two flat surfaces, which is never ideal. I have a feeling that some vendor overstated the ability of their glue to hold these things together through thousands of heating and cooling cycles.
I have not looked at the tanks on the newer models, but I would hope that they have applied some lessons learned from the e46.