I'm replaying Sonic Adventure 2 and it is actually better than I remember. It is the opposite of Adventure 1 which was way worse than I remembered. The Mech Stages are better than Gamma for numerous reasons.
1) No time limit. No dying because the time limit is too short for the stages
2) Better controls. The Mechs feel like mechs more than Gamma did. Gamma controlled like a floaty toy
3) The mechs get their own levels rather than recycled ones. In addition, the levels in SA2 tend to have more platforming than Gamma's shooting gallery levels. In fact, the mech stages in SA2 are far better designed than SA1. They are much bigger scale with actual flashy set-pieces.
4) The lifebar makes more sense than Gamma's rings did
In all honesty Sonic Adventure 2 is a pretty good game. I think the quality of the mech stages are overlooked because of a current trend of the fans overrating Sonic Adventure 1 and significantly de-valuing Sonic Adventure 2.
You can argue if the Mech Stages are relevant to a Sonic game. I fail to see how the stages are in any way worse quality wise to SA1 though.