Donny first a clarification... in most cases "one finger" mode means that when you press one key in the chord recognition area the arranger recognizes a Major chord.

All brands allow the player to add 2, 3, 4, or even 5 keys to trigger more complex chords. Some chord types such as maj6 have to be played in the root position, to prevent confusion with minor 7th for example. Augmented and 9th chords can be played by pressing at least 3 keys.

I personally find major 7th chords easier to play in "one finger" mode. Korgs and Rolands only require you to play two notes (root and major 7th.) I believe Yammies require you to play 3 notes (the arranger plays a full pattern even if you omit the fifth.)

The only chords you can't summon right now in the "one finger" mode are unisons (1+8) or open fifths (1+5). And that's only because the programmed in chord recognition tables don't cover these types. Perhaps someday the manufacturers will add them. Meanwhile I would use Yamaha's "AI fingered" mode, or Korg's "Fingered 2" mode for authentic performance of a song containing these types of chords.

As I see it, the main limitations of single-finger mode are:
-If you WANT to play single note embellishments in the left hand voice, you can't do it in this mode.
-The Yamahas and Rolands I've tested have kind of a hair trigger. If you're sloppy and don't get all the keys down within a quarter of a second or so, you may briefly hear a false chord or the attack of a false bass note before the arranger recognizes and sounds the chord you intended. If you're making a recording, you can go back and delete these false notes out of the score. Korgs have a slight built-in delay that generally prevents this.
-The "short cuts" and rules for advanced chords aren't universal across brands. Roland, Korg, Ketron, Gem, Kawai and Lowrey are similar with subtle variations. Yamaha and Technics use different rules.

Hope this answers your question! -Ted