As I said, the Chain Control Scheme has multiple parts. And, sadly, the Chain is bad all across.
The topic discussed in the fylimar - JandK - Drath Malorn - The Composer - mrfuji3 line of discussion is the "follow the leader" principle. I would distinguish things further.
Party formation. The Chain has no/very little concept of strict party formation. All it has is a vague and imprecise "all characters in the Group follows the currently-selected character".
This is bad. If your movement is interrupted by a conversation cutscene (e.g. you are ambushed by goblins), and you were leading your Group with a front-liner like Lae'zel, combat may start and you discover that Gale somehow got to the front line.
The Chain applies the "follow the leader" principle constantly. Even when the player is not giving any move order.
This is merely ridiculous when you are cycling through your characters to cast healing spells, buffs, etc, and the party shuffles around for no reason. It becomes really annoying when Group members converge on their own after a fight (and this is where Pathing comes into the picture).
The Chain Control Scheme also has other parts (and the post by GM4Him mostly discusses those). I'll distinguish the following aspects.
Selections versus Groups. A Group is a locked Selection. The two concepts are different. Groups can be useful, but the Chain system only uses Groups, which is bad.
In any Operating System, most softwares in general, and most video games in particular (especially CPRG and RTS), you can simply Select several files/objects/characters/units. If I have a "Selection" of the Characters {PC,Gale,Shadowheart}, they are in fact a Group. If I then I select Astarion to do something, and then I select Shadowheart and want to move her to where Astarion is, the Chain system will have PC and Gale follow along because they form a Group. Such a pain in the quality-of-life.
In RTS games, it is often possible to add units to a Platoon (e.g. by pressing Shift+1). Then, selecting this Platoon selected all the units that are part of it. Though units remain individually selectable.
In some softwares likes Inkscape (graphics software), it is possible to select objects, and lock this Selection into a Group, by simply doing Ctrl+G. Then, just like with BG3's Chain Scheme, selecting one object of the Group selects the whole Group. And doing another Ctrl+G unlocks the Group. Importantly, the software knows both concepts : Selections, the intuitive way to manipulate object that every computer user is familiar with, and Groups, which can improve quality-of-life in some cases. Larian's Chain Scheme doesn't.
The player inputs required to manipulate Groups. Adding a character to a Group requires to drag-and-connect their portrait : this is fiddly, and it is more effort than Ctrl+Click. Removing a character from a Group requires to drag-away their portrait : this is sticky, and more effort than Ctrl+Click. The alternative is to do RightClick>Ungroup, which is two clicks and you have to really pay attention. This is a pure User Interface design problem, and it is very poor handled at the moment.
I'll add a couple more points, since I'm already well in, I might as well try to compile many issues in one place (I know, this is pretty useless, even for The Composer, since all of this has already been passed on to Larian long ago).
Group commands. The Chain system is not capable of issuing the same command to a whole Selection or Group.
As mentioned in the first point, even something as basic as Move is such a sad thing that it cannot be properly issued to a Group, which means no party formation other than "one character follows the Move command, the other characters follow the first character".
Putting the whole Group in Stealth was initially not possible. Apparently, it is getting there ... very slowly.
In Turn-Based Mode, when infiltrating a place, if I want to cross a wide open area, I can't issue Dash to the whole group. The only way is to cycle through character and issue the command to each of them.
This is bad, but I have little hope to see much improvement here, because with Larian's Chain Control Scheme, the game simply does not (or perhaps cannot) handle a Group as a group.
The characters do not Jump automatically when exploring around. If I'm running across a chasm on a bridge, I can't simply click on the other side to have my character(s) go there. I have to select Jump, then select the point I wanted my character to move to.
Movement getting interrupted. After I issue a Move command, my character(s) will sometimes interrupt their movement even if I didn't give any other order.
One regular instance is that, as my character moves, so does the camera (especially if I have it coupled to the character), and so does the cursor. If I happen to accidentally hover over an interactable object (e.g. a destructible barrel), my Move command is interrupted. This is really annoying.
Also, following on the previous point, if I see a chasm ahead on the bridge, and want to prepare a Jump command to get on the other side when my character reaches the chasm, my character stops moving as soon as I select Jump, even though I haven't clicked on a destination and thus issued another command. This is an unnecessary slow-down.
Lack of reactivity. If I press Space to activate Turn-Based Mode, the game will take often long and sometimes-precious seconds before acknowledging my input. This is bad when I'm trying to fight the bad Pathing.
Also, when I issue a Move order to a character or Group, there's always a fraction of a second before the character(s) start moving. The movement system really does not feel reactive.
I'm well aware that not all those points relate to the Chain Scheme. As I said above, according to its the title, this thread is about the party movement mechanics. And (party or single-character) movement in BG3 is really subpar. Mostly due to the Chain, admittedly, but not solely.
