Lighter rules require more work from the referee. They need to make more calls, generally speaking. Most referees are there to facilitate the players all having fun. The number of malicious referees is wildly exaggerated. No rule set can protect players from a bad or malicious referee. They will just ignore the bits of the rules that tell them not to do those things. The only constraints on a referee is what they place on themselves and what the table is willing to put up with. Rules light games don't shy away from that. They accept it and lean into it.
This is not true, or rather, not universally true. It's more the general culture the game is supporting. Trad games are already very heavy on GM overhead, so a rules light Trad game will have heavy GM overhead. Other culture approaches to play assign different overheads. Narrative games tend to be much lighter than Trad games, for instance, and offload overhead from the GM significantly.
Take, for example, Cthulhu Dark. If you approach this from a Trad way, as a simplified, GM-driven engine to resolve play, GM overhead will be high. If, on the other hand, you take the game and go for a Story Now approach, then GM overhead is reduced significantly.
Rules light game run a lot quicker. Like a lot. Some still have task resolution systems (where one roll is one round worth of actions or one action), while others have conflict resolution systems (where one roll resolves the whole scene). It just depends on what you're after.
Eh. Quicker is not a universal good. This is true, but not really, independently, a measure of any quality or utility. RPGs are not about speed of play.
Without dozens of books with hundreds of pages each to worry about you can concentrate more on actually playing and trying to tell whatever story emerges from your play that much better. You also have more time to do whatever, even if that's to read up on history, storytelling, genres, or recreational reading.
Here I'll disagree, again. Given you're statement for 1), this means that the GM isn't constrained and can do what they want to do with their story or setting without having to check to see if it aligns with the rules. The problem here is predictability. The rules of an RPG give the players that ability to successfully judge the stakes for the play. If you take that away, especially if you replace it with the GM, then its harder to predict those stakes and make good decisions. Again, this really comes down to a culture of play issue -- if you're Trad or Neotrad, this is less of a burden because play is still focused on exploration of the GM's setting or story so stakes are communicated as needed for those moments. It's anathema to gamist play, which is why you don't see gamist rules-light systems very often (and I can't immediately bring one to mind). Story now play deals with stakes as an ongoing discussion anyway, so little changes on rules density.
There's a good reason what the vast majority of rules-light efforts are Trad or Story Now approaches. Most of those are still Trad, because that's what the current market gorilla does.
