Plastic external enclosures are hotboxes. Users often report temperatures of 50C or more.
In addition to the integrated USB bridge electronics on the HDD PCB, the Passports also incorporate AES hardware encryption, whether or not you have set a password. That may or may not be desirable.
If you build your own external drive, then you may have a longer warranty. Moreover, there will be no warranty issues if you need to remove the drive for any reason.