The skill gap
Greetings!
I'll try to not make it a wall of text again, so I condense it as good as possible.
I had a topic like this already back in season 7/8. I noted that I - a casual and averagely skilled player - usually get paired with people of a different skill level, both ally and enemy. There were times in which I got paired with two new players who of course played like a new player would, but I also got times in which a single guy in my squad ran ahead and murdered squads one after another, winning 'our' match with him having 10+ kills. This isn't limited to allies of course. With enemies, that's the case as well.
This enormous skill gap is especially noticeable in Arena mode now because you play against the same enemy for a minimum of 3 times. And the difference between players is astonishing. It will either be a massively worse team or a massively better team. An in-between rarely exists, and especially the in-between yield the best fights.
And while people might think that I'd say "fighting against bad players is fun, fighting against better players is frustrating", it's actually both sides that are frustrating to fight against. Players with a much lower skill level feel like the NPC's of a generic shooter game on easy mode, while people with a much higher skill level feel like terminator reloaded. And this makes me doubt that there is a significant SBMM.
So in other words, what currently frustrates me the most in Apex Legends is - aside from the still ongoing spitfire meta - the huge skill gap, which shows no respect to either side of the skill spectrum. Good players get paired with bad players, with the latter usually being more frustrated.