For example when you are using it, your 5th and 6th party members seems to be ignored by any in-game event that affects your party.
Not true. All "quest triggers" work regardless of the number or order of companions, for a start. And several party banters can involve up to several companions at once.
What happens on the other hand is that in SOME circumstances the game won't let every companion you are carrying react to what you are doing, but just pick one of them to comment on it (seemingly randomly).
To be clear this happens even with a party of 4; the party of 6 just makes it slightly more noticeable.
There are few other quirks that should be ironed out by modders if Larian won't do a thing about them (and I have no real reason to think they will).
- during dialogues companions tend to have a fixed, pre-established position in the background that DOESN'T account for additional characters, so they often overlap now.
- there are some occasional "bottlenecks" hardcoded into the game where you either ungroup the extra party members temporarily before doing something (i.e. taking the boat in the Underdark to reach Grymforge) or they get automatically "killed and glitched away".
- the DEFAULT formation for the party absolutely fucking blows, as it's this awkward, inconvenient widespread triangle, and I'm not sure if modders will be able to help in this sense (see the screenshot I posted above).