A simple example, one player has a cleric who can heal the team between fights and the other player plays without him (technically in 5e nothing prevents you from playing without cleric).
One group will need to rest more often than the other because of the damage they take.
The second group can make use of: healing via hit dice during short rests (assuming Larian increases # of short rests), potions of healing, possible spell scrolls of healing depending on how Larian implement that mechanic, healing provided by any of: a bard, a druid, paladin, the fighter's second wind and/or monk's Wholeness of Body. Arguably many parties without a cleric will take less
damage during combats because they'll be killing things faster.
Even if certain party compositions have to rest more and thus have a more difficult time, that's ~fine because it's for an optional ("realistic") difficulty setting.
That said, I don't see this working with the Base Camp in BG3; either you'd still be able to go to base camp between these campsites but for some reason be unable to rest, or you'd be locked into your chosen companions until the next camp location which would be frustrating.
Also, as @Rhobar mentions
Was the checkpoint disposable?
If so, what would you do if the fight went wrong?
If they are not disposable, what will prevent the player from going back and resting? Is the solution to respawning enemies like in dark souls?
there's the issue of checkpoint disposability. Either players can just walk back to a previous rest spot or every single campsite must be single use, which may be a bit much.
OP, I think your idea is good for select areas/dungeons, combined with the inability to teleport to safety. Like the Hag Lair - you shouldn't be able to long rest in the Hag Lair or teleport out, and if you manually leave then something
happens. E.g., Mayrina gets sent off to the other hags, Ethel recruits more guards and prepares an ambush for you, etc. Change of content - not loss of content. For longer dungeons (again where you can't teleport out), it'd be reasonable to find a safe campsite within the dungeon and be temporarily unable to change your party while in that dungeon.
For regulating long rests throughout the entire game, simply regulating food required for long resting is probably sufficient. Perhaps combined with requiring players to manually walk to any teleportation sigil, which incentivizes but doesn't require pushing onward toward the next sigil.