i like the Adventure games, possibly love them.
but i don't defend them, i know what they are.

it all started around 1995. your only options for "new" Sonic games were 3D Blast or buy a Saturn just to play a little 3D segment in Jam.
Yuji Naka wanted to replace Sonic with an IP he had more control over, and so personally cancelled at least 3 different Sonic games we know about, in order to force SEGA to push NiGHTS. it didn't work.

so 1998 comes, and SEGA unveils their new "Sonic RPG," now called Sonic Adventure. because they realized abandoning their top selling, world famous mascot to the dumpster was actually a retarded idea.

but they waited too long. Naughty Dog had already eaten their lunch with the easy layup. and of course Mario had dominated everything and one. so they just threw everything in. that's why Sonic Adventure is a little bit of everything.
Sonic Adventure was born the moment someone in a room full of SEGA execs said
>Quick, what's a recent game we made that's doing well? SEGA Bass Fishing? Ok, put it in.
they didn't know what to do with 3D Sonic so they did everything. you've got a Panzer Dragoon segment, kart racing, whack a mole, minigames, all kinds of stuff.

i don't hate Sonic Adventure 2, but i do think 1 is the better game. everyone had already heard the word that the Dreamcast was getting axed and SEGA making an exit while development was still going on. so they knew it would be the end. they were still putting out screenshots after SEGA's announcement, the game didn't release until months later.
i think you can see the team's knowledge of all this reflected in the design. fewer gameplay types, less overall content stretched out for longer (way fewer minigames, no exploring HUBs, have to do levels 5 times instead of 3).

Sonic Adventure was SEGA throwing spaghetti at the wall.
Sonic Adventure 2 was SEGA looking at the biggest chunks that stuck and trying to make a whole game around just those.