I'd say they're charismatic and funny, but irredeemably bad people. It was refreshing that the show didn't shy away from that; in lots of comedies, the characters are basically psychopathic if taken literally, yet we're still supposed to like them and to see them as having hearts of gold if they make the occasional nice gesture. Always Sunny just leaned hard into portraying them as terrible people who were only 'likable' in the shallow sense needed to make the show fun to watch rather than an ordeal.
But I think the creators eventually lost sight of that -- I remember the big serious episode they did with Mac's dance, and I just find it baffling because in order to buy into the emotion we were evidently supposed to feel, we needed to take the characters seriously. And as soon as we take the characters seriously we are (or should be) overwhelmingly aware that we're watching people who have proven over the previous umpteen years to be irredeemable sociopaths, which kind of takes the edge off the heartwarming pride story.
I would also say that the Always Sunny gang really aren't sympathetic either, but it's a para-social trick of having spent so much time "together" with them over so many episodes.
I suspect a new viewer coming to watch the latest series of IASIP would not see them as sympathetic. That's quite different to The Office (US), where a new viewer skipping to later seasons would not have the same opinions as a new viewer watching season 1, where Scott was much closer to a Brent type character, before he was redeemed and made more pitiable than awful over the seasons.