I recently bought an NTSC-U Dreamcast (Revision 2, unfortunately), and it plays CDs and Space Channel 5 just fine, but immediately returns to the Dreamcast menu after displaying the Sega logo upon booting Sonic Adventure. The seller I bought the game from on eBay says they tested it in their Dreamcast, and that it worked fine. They also stated that it was an NTSC-U disc (I had asked them to confirm, just in case).
The disc certainly looks official, even stating "PRODUCED BY or UNDER LICENCE FROM SEGA ENTERPRISES, LTD." and "TRADEMARK 'SEGA'" (with "SEGA" being printed as its logo) on an inner ring on the underside of the disc. It appears to have some light scratching in a few spots, but still works enough to boot into the logo. I even tried gently wiping it with a lint-free cloth, to no avail.
TL;DR: The Dreamcast works, the disc works, the regions match, and the disc seems to be legitimate; but the Dreamcast goes straight back to the menu after booting into the disc to the point where Sega's logo appears.
The Disc: https://imgur.com/a/2rXyVxY