It's more that groups have a relatively clear objective and that people are less inclined to overstate their readiness.
Disband conditions are clear. E.g. it might be 20mins to clear exdeath or disband. Or it might just be "early disband possible"(which is a more vague description that just means that if at any point after the 15min cutoff people feel this isn't going to work, a vote will go up, and 90% of the time it will pass). Or there is a clearly stated time. Whatever those conditions are though, people will stick it out.
There are still people that overestimate their ability, but it's nowhere near as bad as what I hear from other servers. Using RF for savage is common and generally people using it are more confident in their abilities than with PF. RF tends to default to disband at 15mins if there's problems, maybe a couple more pulls if it looked like unfortunate accidents. All of this comes down to a more team-play mentality though where people won't join a group they feel like they would be a burden in. The flip-side is if someone bad does join, everyone will just grit their teeth and keep going, kinda wasting everyones time. I don't think I've ever seen a player get kicked from a group
The min-maxing bit is more or less accurate. Most people have ok dmg, but it's generally below 50% of fflogs aggregates. Which means that on O4S that there is actually not much breathing room for damage(I've been topping dmg in my static as a mch, with most of the group under 4k). In general JP is more opposed to ACT.