The unimplementedTassert helper that is use in most function definitions in VirtualCollectionImpl has a few problems:

• It is not marked with MONGO_COMPILER_NORETURN, so each function has to return something.
• The tassert code is not unique, so it is not possible to tell which function was called.
• A backtrace is not generated which means it is impossible to tell which unimplemented function was called.

MONGO_UNREACHABLE solves these problems. I also don't believe that calling unimplemented functions is expected to be a common problem in our code that we need to use tassert to protect the process from crashing.