but not from the sharing, but rather because XP award itself is unbalanced (well... it's balanced in that assumes the base builder isn't getting all that phat looting/scavenging XP or the combat is away from the base).
It is not really well balanced. And it is very hard to balance because there is singleplayer and multiplayer. If you want to keep multiplayer all together you NEED to share every XP (divided amount among the players, so a 400xp kill doesn't earn every player 400xp, but with 4 players 100xp each, with 10 players 40xp each) and not even looking at the distance. But that's kinda boring imho. Each players progress imho should depend somehow on what he is doing in the game. And the afk-dude than gets outleveled by the others... to name an extreme form.
The thing is, if you balance the XP gotten by specific roles for multiplayer ot be equal, like e.g. in our last quests my mate cleared the pois while i went behind and looted, one person in singleplayer doing both roles would get a massive amount of XP and therefore progress very very fast. It's basically impossible to balance it to fit both MP and SP (without any form of XP sharing).
I also don't understand what the massive problem of some people is if they have different levels. Yeah, the lvl50 player has more advanced skills than a level 30 player. But skills is not everything. The lower ranked player still take profit out of the better skills of the advanced player, because he can provide him better weapons, better armor, tools, mods, etc. There is no item you can't use because of missing skills, you just can't craft it. But with more advanced teammates you get access to better items, than you probably would have without them.
Imho the XP sharing for kills is fine. Mining and building xp don't need to be shared but should be lowered. Kills from traps should be shared without a skill and behave like a kill from a third player. Everybody in the near area should get like 50% of a trap-kill. Independently on who placed the trap. So if a zombie dies in some spikes of a random poi, you will get some XP anyway.
In my latest playthrough we can't even really play quests together, because i'm the dps-dealer-tank-build and would like to rush through the buildings with my shotgun, waking up every zombie intentionally and let them come to me, while he is the agiltiy-sneaker, who has a hard time if he accidentially wakes up 5 Zs at once. You could say, he wouldn't survive my playstyle long.

So i let him sneak in advance, while i loot behind him. And if he enters a auto-aggro-room, he retreats asap and calls for my help.
Other way round is not really working, because if i wake up everything, Zs often may alsocome from behind and overwhelm him while looting too.
Yep, so in the end it is always a game, where you need to arrange yourself and think about how to do it, and not expect everything beeing equal in every case.