Man, this crossover trope is a mixed bag for me. You've got stories where the League shows up at a gala and sees the Bats in their natural habitat—that "found family, Wayne family secrets" dynamic can be a delight when done right. But the best ones, honestly, flip the script. I'm partial to fics where the Justice League slowly pieces together that Batman's terrifying network of operatives are, in fact, a bunch of traumatized kids and young adults trying to do good. The dramatic irony is delicious. A story called 'Of Bats and Broken Things' handled that reveal beautifully; Superman's growing horror as he connects the dots between Robin's puns and the kid's scars is something I still think about.
Where a lot of fics stumble is making the Batfam either too OP or too soft. The tension works when the League sees them as a potential threat or a tragedy, and the Bats themselves are just... trying to survive Tuesday. The real gems explore that culture clash: the League's galactic-scale morality versus Gotham's gritty, no-win scenarios. I tend to skip the ones that are just fluffy team-ups with no edge; it feels out of character. Give me a fic where Constantine shows up, takes one look at the Batcave's supernatural wards, and just nopes out immediately. That's the good stuff.