Hi Bill, you've asked a good question, unfortunately the answer is a long read...
Chord Intelligence enables playing the most common types of chords with a minimum of keys pressed, and a minimum of hand movement. Majors can be triggered with one key, minors can be triggered with the third above OR below; and four-note seventh chords can be played by pressing only two keys. It's a skill multiplier that makes it much easier to play smooth progressions.

Intelligent chords are a SUBSET of the full chord, so a progression will NEVER be more difficult than it would be on a piano or organ. You never have to play "extra" notes that aren't part of the chord, yet it does not penalize you for playing all of the notes if you're used to doing so (i.e., no re-learning). In the worst cases such as Augmented, Sixth, or m7b5 chords, you do have to play all of the constituent notes. Thankfully these chord types are less common in the music I play.

One thing that makes the Roland system good is that Bass Inversion ("Leading Bass" on the Ateliers) is a separate control. When you activate it, you can STILL play major chords by pressing one key (which triggers the root bass.) You can still take advantage of simplified fingerings (such as C2-Eb2 = C-minor/C bass). However, when bass inversion is active, the lowest note played determines the bass. So Eb2-C3 would give you Cm/Eb; Bb2-C3 would give you C7/Bb, etc.

This same fingering was used by GEM and the Hammond XE-series, which are now unfortunately out of production. Ketron, Casio, and Korg employ the same concept, however there are subtle and often unpublished differences which I have uncovered by experimentation and direct comparison that IMO make them slightly inferior, and require re-learning "muscle memory."

For example: on the Ketron SD-series, the most convenient 3-note inversions for minor seventh chords tend to be recognized as a sixth chord, which "gets in the way." [Rolands require FOUR notes in root position to trigger a sixth chord.] Casio gets around this issue by having a mode that disables 6th chord recognition altogether!

Like Roland, Casios have a mode in which the lowest note played controls the bass, while still permitting other one- and two-finger shortcuts. It's good; fingering conforms about 95% to the Roland system. However, in its latest generation of arrangers, playing a fifth below the root, i.e., G2-C3 doesn't change the chord. To get C on G bass you have to reach up and add the E above. Personally I think this is an oversight by the programmers, but inconvenient nonetheless. On Rolands, two notes suffice to trigger C/G.

Korg has more two-finger shortcut intervals than any other brand. One of them, the augmented fifth, was implemented 26 years ago with the i30 but it's never been documented in a manual! The problem is, when you're trying to play on-bass chords these shortcuts get in the way. For example, even when Bass Inversion is activated, E2-C3 gets you E#5 (E aug) instead of a more common C/Eb. To get C/Eb, you have to add the G2 in the middle, so you're basically playing the full chord. G2-C3 gets you Gsus4, to get C/G you have to add the E3 above, etc. These unwanted behaviors could easily be suppressed with a menu option but have not been.

Yamaha uses a different paradigm, bundling the "on bass" functionality within their own intelligent mode ("AI Fingered"). IMO this is less flexible than having bass inversion as an independent setting. Under this system, many 2-note combinations are interpreted as slash chords. The root bass can always be obtained by playing ANY 3-note inversion. However, it's not always possible to obtain a desired slash chord directly. For example: if you want A7/G and you play A below G, you might get Am7/G depending on the prior chord(s). You can't even "force" Am7/G by playing G2-A2-C2 because that would give you Am7 with root A bass. What's worse, playing a single note results in a thin-sounding Unison (1+8)! This is no help at all when you're used to playing major chords with one finger. I can't even play "Happy Birthday" without re-learning years of muscle memory! What were they thinking!? Before my last Roland arranger croaks, I'm going to commission a third-party utility that edits the CASM of Yamaha styles, adding a third and fifth to all style tracks, so that you hear a full major chord pattern whenever a unison is recognized by the arranger!

One advantage of Yamaha's approach is when a song contains a mix of slash and basic chords, you can use the 2-note fingerings for on-bass and play the other chords in any convenient inversion without changing modes. But there IS a workaround for Rolands that restores much of the lost flexibility, which I'll detail below...

Yamaha's approach also raises the issue of how to play "incomplete" chords. I personally have no use for Unisons, I've never seen them on any of my score sheets. But if you want to play heavy metal or a song like 10cc's "I'm Not in Love," these won't sound right without the open fifth "power chords." You can get this on Yamaha or Casio by playing 1+5, i.e., C2-G2. Unfortunately in Intelligent mode, Roland recognizes this combo as C-major (which is a waste, because you could also trigger C-major by playing C2-E2, or C2-E2-G2, or just C2 by itself!)

The workaround is pretty crazy, but there is one... Roland stores the chord recognition mode as part of a registration. By using a combination of outboard MIDI Solutions gear and specially configured registrations, I have my BK-9 set up so that when I hold the left footswitch down, it shifts into "Standard" mode allowing me to play incomplete chords like open fifths, C7 no 3rd, etc. As soon as I release the footswitch, it reverts to Intelligent mode. [Some Rolands have a native function to switch between Piano and Standard modes with the press of a pedal. Unfortunately their engineers didn't envision my use case for real-time switching between Intelligent and Standard.] You would have to take a similar approach (summoning an alternate registration by foot pedal) to play incomplete chords when using intelligent mode on a Ketron. Due to a dumbing-down of the registration controls, I'm not sure this workaround is possible on any Korg made since the Pa3X!

A similar workaround makes it easier to play songs that consist of mostly basic chords with the occasional slash chord thrown in. I'm also using outboard MIDI Solutions gear to turn bass inversion into a "momentary" function (i.e., only active while the pedal is pressed.) As soon as I release the footswitch, chords may be played in any convenient inversion. This is built-in functionality on Roland's FP-series style pianos and entry-level arrangers sold in the late 1990s. However on the BKs and E-A7 it's implemented as a latching function which requires two pedal presses. Depending on how many slash chords there are in a song, my momentary real-time approach with a foot pedal is easier than leaving Bass Inversion on for the whole song. IMO it's easier and more consistent than Yamaha's implied mode switching, because among other things, AI Fingered doesn't allow one-finger Majors (unless you were to edit the style CASM in the manner I described.)

The bottom line is I come from a chord organ "one finger equals a major chord" background. If someone learned on a non-intelligent instrument and is already used to playing 3- and 4-finger chords, the subtle differences in these systems fade into the background, except for the possible tie-up between the minor seventh and sixth chords. All brands of arrangers should allow more customization with regard to chord fingering.

A couple other considerations: I'm strictly a left-hand chord guy. But Roland arrangers allow you to configure either half of the keyboard for any type of chord recognition, in any mode. This is a nice convenience if you want to play the bassline with your left hand, or with foot pedals. (On some other brands, you would have to fool the OS by feeding MIDI output back into the board, or by using a second keyboard controller, etc.) Diki plays pianistically and he has offered a lot of praise for Roland's "Pianist" (whole keyboard) modes, compared to other brands. I use my RH strictly for melody and wouldn't know what to do with a sustain pedal. So my treatise above doesn't address the nuances of full keyboard recognition across brands. Perhaps Diki will weigh in, or search his posts to find out why he thinks Roland does a better job than the others with Pianist, or "whole keyboard" chord recognition. If you made it this far, thanks for reading!!