I am also having this problem with NSSecureCoding. I suspect it's a problem with existentials. I agree that it is a bug -- they should be hidden behind the Codable interface.

I had a working app in Xcode Beta 2, but now in Beta 5 I am seeing a very similar issue. I am creating the ModelContainer from a url which points into an AppGroup container. The app hangs for a long time before crashing with a segfault EXC_BAD_ACCESS (code=2, address=0x16f96bff8). The fact that it's a segfault makes me think that something is happening on the wrong thread, as it should be some more safe error if there is e.g. a migration issue with the SQLite storage.