RGL PUGs are like the first, most obvious way to get into competitive at the moment. I've seen a bunch of people add up who've never played 6s in their life, and I think it's reasonable to be frustrated that someone on your team doesn't know how to play (though you shouldn't be flaming people for being new). I don't think it makes sense for players like that to be able to wander into higher level PUGs. Brand new 6s players should be playing newbie mixes, TF2Center or in the case of RGL, the Open PUGs. The Captain PUGs are built around the idea of the players actually knowing each other and trusting each other's skill level, but when you have brand new/amateur players in the same lobbies as Invite players, there's gonna be a huge disconnect in not just skill level, but also banter and familiarity.
It's cool that you can get in games with top level players as a mid level player (and if I add up to an Invite PUG I know what I'm getting myself into), but there's not much benefit and a lot of drawbacks to letting a brand new player get into the same lobby as an Invite player. It's just a recurring toxic cesspool for the new player, if they even choose to re-add; I've definitely felt the cycle of being flamed for being bad, yet still adding up because I feel like it's the only way to play/improve, and this has happened even before RGL PUGs.
I think Captain PUGs currently have like a 1200 Elo requirement to get picked? I feel like increasing that requirement so that people need to have at least some sort of established skill rating would be a good way of increasing the quality of Captain games, as well as encouraging new players to play Open PUGs (or pointing them to resources like the Newbie Mixes or TF2CC), which are designed to automatically balance and pick PUGs. The 6s player base is way bigger than its Advanced-Invite community, yet those are 90% of the players I see added up to PUGs.
I avoid Open PUGs because they're never populated and I don't trust the match quality, but at the same time I've always wanted a system for 6s that was similar to other competitive games, where I can just click a button to get automatically matched into a game. It's kind of a chicken and egg problem where Open PUGs suck/are dead, but maybe as a community we should look to be pushing a format which is more friendly to newer players.