Plasticard is well justified because it is easily worked and long lasting. If unpainted, it will de-nature over time, but it is basically the same stuff that plastic kits are made of. Of course it is flexible. So is plywood. It is brittle in that if you bend it far and often enough it will break. So will everything else. It also comes in various grades, as has been noted in the past by people who made their own vac-form machines. When built, the model will have a surface that needs very little preparation for painting.
I would not consider making a hull out of it, though. Something pre-moulded is OK, but to build it up from sheet, no. Other materials do the job so much better, but decks & superstructure where generally you want looks and lightness rather than strength, it comes into it's own.
For those with the required skills, Jörg's method with brass sheet will be light, and probably everlasting - just a question of learning the new skills and probably much practice.