The main reason why I think MAD should be see as false to everyone in the mainstream denominations is because it is a denial of life in the Spirit walking by faith. This is key for me, regardless of whether there were two gospels.
When I first cottoned on to what was lacking within MAD theology, it surprised me bigtime, because the message of grace within MAD preaching is faultless. But something just didn't seem to sit right, and it nagged away at me until it twigged.
No repentance, no faith, no life in the Spirit.
It's a theology that sympathizes with the natural / carnal man. And so the grace message within MAD is all about disarming the power of sin through the law, in order to settle the flesh down to where there is no longer any obligation upon the believer to be anything, do anything etc
I came down pretty strong on all the madists, because we regard this kind of sympathy for the flesh, instead of overcomming power in the Spirit, as being little more than gnosticism. However, I've been thinking on this recently, and if this is the only way that madists can feel free from legalism, they will fight to the death for what they regard as being justified. The problem is, the justification within mad i see is actually justification of the flesh, not spirit. This is where faith is lacking.