IIRC, accordians have chords on one hand and single notes on the other, right? So playing chords doesn't prevent you also playing a tune or doing anything else more interesting (eg. moving bassline) with single notes. And with an autoharp, you've got a few dozen strings, so it's perfectly possible (if you're good) to play chords and melody together.
A guitar only has six strings though, and if you want to play a chord then by the definition of what a chord is, that removes at least three strings from play melody-wise. If you want a 7th chord, that's at least four strings out. So anything which tried to play chords for you will effectively prevent you doing anything else with the guitar. No moving basslines, no melody, no harmony, just a monotonous "chung chung chung chung".
Also, autoharps and accordians have one fixed note per string or per reed (or two if your squeezebox plays different notes in and out - either way, it's fixed). On a guitar though, each string lets you play up to two octaves of notes, with the only limit being what you can hit simultaneously with your hand position (and even then, two-hand tapping gives you further options). The ability to play chords in different places gives different "flavours" of chord by virtue of the chord inversion, pitch and string choice (the same note on different strings sounds different). Even with standard tuning, there are literally dozens of ways of playing a G chord, for example, all of which sound different. There's no way on earth that a mechanical gadget can do this.
And we haven't even touched on the possibility of altered tunings, at which point any fixed chording system is totally knackered...
Sure, beginners are often only capable of "chung chung chung chung", but that's just a transitional stage on the way to better playing. Having some gadget forming the chords for you will prevent you *ever* getting past that beginner stage. In other words, McGrath's fabled virtuoso simply won't appear, because virtuosity is by definition excluded from such a system.
Of course, you could have a system of 24 buttons per string which would selectively fret individual notes on each string. But then you might as well press the strings down yourself, right?
I don't think this falls into the same category as capos. The only people who sneer at capos are some classical players, and that mainly for reasons of tone. Everyone else (including flamenco players) sees them as the necessary tool that they are.