The issue wit the Major costuming clubs and EVA Foam is not Fragility, but durability and length of life. One of EVA Foam's strengths is its flexibility and ability to compress, however, that becomes an issue with paint. You can prep the surface and get a good paint job, but, over time, sometimes even the first time you wear it, that flexibility will causes creases and cracks in the paint job. Plus EVA foam will start to deteriorate and break down the more it is flexed, as it will when you wear it, and it can hold can absorb sweat which causes an odor issue. All of these are not really issues if you make an EVA Foam costume and wear it a handful of times to Conventions, but some active Garrisons/Bases/Clan in the Major Costuming Clubs have the opportunity to do literally hundreds of Events a year, and a member can attend as many of those as their personal life and schedule allows, which will severely compound those inherent EVA attributes.
But this is not the place to debate Costuming Clubs Rules and Operating Procedures. They are private organizations with every right to make their own rule, just like TheRPF might have rules some people don't like, it's also their right to have their own operating procedures. If you don't like them ,you don't have to join, just like people don't have to post here if they don't like the rules here.
As for your issues with Sintra, from the way you describe it, I am almost led to believe you aren't actually working with Sintra. Not being able to Super Glue it, and it "Shattering" when cut it, plus it taking a long time to cut, that sounds like Acrylic/Plexiglass, not Sintra. Sintra should cut just as easily as EVA Foam with a Sharp Clean blade, and it should super glue together super easily, and I have never seen Sintra shatter while cutting it.
Maybe some more details, and even some pictures would help.
