I was a child when Sonic Adventure first came out on the Dreamcast. I was also one of the rare few who got to see it new and firsthand. At the time it looked impressive. Now with hindsight, it’s basically the Yak-38 of video games.
The Forger was basically a tech demo of a V/STOL fighter that got shoehorned into being an operational aircraft out of desperation. It was horrendously underpowered and unsafe. Likewise, this is a massively erratic way to show off all the things the Dreamcast could do more than an actual game. Sonic himself is a barely controllable pinball. Everyone else is there to represent something “new” and “amazing”. Tails can fly. Knuckles is there to have the same kind of collectathon gameplay pioneered in Mario 64. Amy-uh, does, basic puzzle stealth? Gamma the robot does third-person shooting by way of locking on, and Big the Cat infamously has that classic element of a speed game: Fishing. Slow paced fishing at that.
The cinematography in the cutscenes is utterly horrendous with the slightest point of comparison to anything else. And this introduced the storyline elements that would explode to horrendous proportions in Shadow and 06 and remain with the series even to this day. Which is to say, a combination of mystical mumbo-jumbo, Dr. Robotnik/Eggman messing with something he shouldn’t, and tons of new characters with each installment.
What I consider interesting is that Super Mario 64, made by Nintendo from a position of strength, did not do anything like this. It kept the same basic excuse plot as the past installments, and didn’t feel like it had to push anyone new very hard. Sonic Adventure, made by Sega from a position of weakness, had to stretch, and it failed in that regard.
The tragedy of this for the series was that instead of trying to improve the fundamental controls, Sonic Team focused on one gimmick after another. Mechs, teams, guns, telekinesis, anything but razor-sharp platforming. Adventure didn’t cause the famous 3D pit all by itself, but it started the process of digging.