I am intimately familiar with the difference, I own them both systems:For acoustic accordions, free/converter bass and standard bass instruments are quite different since the need to carry a melody on few reeds requires a more cutting disposition. Also there is less space to work with: a 5-voice standard bass has 60 reeds, a 2-voice free bass about double.
Roland doesn't differentiate here as far as I know (which admittedly is not very far).
Also on an acoustic accordion, converter/free bass opens a single pallet on a button while standard bass mode opens two pallets (one of them large) on the bass buttons and three pallets on the chord buttons. That makes for significantly different resistance/action in single-note mode.
Hohner FB-36

Hohner Morino VI N

Roland FR-8X

Hohner Gola 459

For me, it has nothing to do with the internal workings, but rather because the feel of the Roland is very uncomfortable to me. I started on the MIII system in 1973 (50 years ago, holy crap, that makes me feel so old... LOL), and choose to stick with that today and the spacing and placement of the buttons on the Roland gives me craps after a few minutes of use, it's no more difficult than that. Since it is not comfortable, I don't use it ever for Free Bass music on the Roland, though I do use the FB "switch" with access to percussion instruments on the left hand now and then.
This is strictly me, has nothing to do with how good or bad the other systems are nor what makes them different.
Another one of my idiosyncracies is that even if I equally enjoy a digital and acoustic, I have a strong personal preference for the sound of the acoustic/Free Bass combination over the option of Free Bass/anything digital. There is something distinctly pleasurable to me to be able to play piano music on an acoustic accordion, whereas I can play other music that I like on the 8X and enjoy all the sonic benefits of a digital since I have access to both easily enough.
I've said it before, but I think that I am the most perfectly balanced digital/acoustic player ever known, I have no distinct preference of one over the other, Each holds a special place in my heart, neither better, neither worse... lol