I was talking to a friend about this last weekend and it’s still a little on my mind, so I wanted to get it down—basically, we were talking about how in LARPing and gaming there’s always someone who wants to try and cheese the mechanical system in a way that’s not appropriate, and I also posited that there are ways to have powergamers in rules-light systems, too.
(Admittedly, the presence of twinked out characters in both kinds of games is something of a failure on the part of the STs/GMs/whatever they’re called; being able to say “no” to a character build should always be the prerogative of the running team.)
Unbalanced characters in rules-light systems only become a problem if the gamerunning team isn’t paying attention to how narrative leverage is distributed; it’s true that you can have characters of all backgrounds if and only if the gamerunning team considers the balance of “how much influence can a given character have on the narrative.” Where you get into trouble is when adjudicating whether a character can or cannot do a thing comes down to “whether they could reasonably do [X] easily based on their background.”
I’ve seen a lot of characters in my time in systems that were relatively rules-light who were allowed to get away with quite a bit just based on their character pitch. (I am not entirely innocent of this.) When a character can say, “well, my character is over a thousand years old and studied at the library of Alexandria, can they have heard of this—” or “my character has an extensive military background fighting monsters, can they know the weakness—” and the storytellers say yes because it “seems reasonable,” that’s unbalancing the game unless they keep careful track of how often each character is allowed to draw on their backstory for expertise.
I suppose the other thing is that when you allow people to put wacky-ass shit in their backstories for whatever reason, the other way this makes the game unbalanced is that anyone who didn’t do that and is playing a mostly-reasonable character feels compelled to defer to the character who has this over-the-top stuff in their backstory because it Makes Sense, which can happen even without a storyteller allowing anything actively; it just becomes passively accepted that a character is The Shit and should be listened to, which is IMO not great. The thing about LARP is that stuff that could get a pass in tabletop now has to contend with the scale of 25+ players, and becoming much more of an ensemble cast show.
My sort of general recommendation for solving this problem is part—stick to the mechanics you have, and if you’ve got something like a fate point mechanic for influencing the narrative using character aspects, by God, use that fucking thing; the other part is feeling free to ask players to tone down or cut aspects of a character’s backstory when it gets out of hand on pain of haivng to submit a new character. Having to be harsh or mean about something sucks but I can say from experience that it sucks way more when you’re having to deal with it in the middle of a campaign when there’s no takebacksies.