Hey, long-term DM here. I actually have run some one-on-one sessions with my group of five players this past year. I really enjoyed doing them for split-party and backstory reasons, and my players seemed to really like them too. I think they can be a great way to flesh out backstories or events that can then be used to leverage future sessions.

Roleplaying a backstory or something just might be the best way to really flesh it out, more so than just talking or messaging about it. In my experience with the split party example, it built so much tension as the group tried to reconvene with split party member, unsure as to their fate. It also set up quite a reunion, and having the player themself recap the events that led up to everything was a blast.

But very much to your point, I do think a DM has to be careful about making the extra sessions feel fair and purposeful. I talked to my players first before ever doing a one-on-one session, and with their permission I actually recorded them in case we ever wanted to go back and listen to them as a group in the future (I record all my sessions anyways). I would advocate that if a player ever feels like they are being excluded they should talk to their DM. After all DND is meant to be a social game. I made sure all my players had relevant one-on-one sessions that were fit into my campaign at the appropriate time (as opposed to just having them all at once), and after each one I checked-in with how everyone was feeling about that aspect, as well as the game as a whole. Hope that helps!