He's just a Sonic re-skin only available in his levels, and 'Metal Sonic' isn't even available in the Adventure mode.

Metal Sonic is the only thing you get for collecting all 130 emblems in DX, and he doesn't even have his own unique animations or gameplay.

The wrong sound effects play often, and some don't play at all.

While the mission mode is an interesting implementation in the game, several missions are boring and repetitive, an example is where you have to move a statue to its objective or excavate a specific part.

The concept of skipping the scenes of the game is interesting but it does not run very well, there are times when you have to press the Start button a lot or at a specific moment, sometimes if you skip a scene at the time wrong it sometimes crashes or freezes the game.

Somehow, some of Knuckles version from the 1999 E3 demo version made it into this version.

In one scene, Amy's model from the original Dreamcast game is reused, so she looks inconsistent and excels from the other models.

Also these newer character models do not fully fit in the game, as there are too many times when a graphic glitches, for example, Sonic's shoes when he runs show incorrect textures, also sometimes Sonic's eyes disappear, which looks weird, and the characters lose a lot of expressiveness, so in several scenes the intention is lost, for example, in Tails' story, when Sonic rescues Tails, he doesn't look at his friend in a cheerful way, he just makes a meh expression.

The newer character models, although are higher polly than the originals and have actual hands, look like plastic due to the bad lighting and clash with the other characters (most visibly, the echidnas in the past).

Inconsistent framerate the game will constantly dip below 60 FPS and even 30 FPS at times (this isn't the case on the Wii or emulators, though) in addition, several objects didn't have their logic adjusted for the higher frame rate, and have incorrect behaviour.

All the environments in the game (except for Emerald Coast, Lost World and Mystic Ruins to some extent) are overall more bland and uninteresting to look at.

Several textures now have compression artefacts that ruin the details (this was also the case in Sonic Adventure 2: Battle, but to a lesser extent).

Transparency is also a major issue, with the workarounds making textures look blocky and low resolution.

One of the most note worthy changes is the lighting, which looks more dull than the original and loses some effects such as Sky Base getting darker when beneath the clouds.

Despite coming out in 2003, the graphics manage to look worse than the original game which, by the way, came out in Japan in 1998.

Most infamously, the walkway to the loop in Emerald Coast has broken collision detection.

Due to being rushed, Sonic Adventure DX is a lot more glitchier than the original Dreamcast version it came from.

Sonic adventure dx pc box art