It takes a lot more force to hold a string down than open a valve. A mechanical guitar-Stradella system could mean that one finger was trying to depress six or even twelve strings at once. Making it both reliable and light enough not to induce instant osteoarthritis of the finger joints might be tricky.

An electromechanical system might work but would be heavy. Probably best adapted to a pedal steel.

The other issue is complex chords. Guitarists (well, jazz guitarists anyway) are used to playing chords with up to six pitch classes present. On an accordion you can get that by combining buttons. A guitar mechanism would need separate buttons for each chord type, and by the time you get up to all the variants of 13th chords and the like, that's a lot of buttons.