I think it's non technical things, like the layout and UX of browsing/finding Lemmy communities and posts. Lemmy communities are kinda hidden in the small World icon, they aren't first class citizens (not saying they need to be prioritized above the local NodeBB categories, but they're kinda pushed under the rug)

It's been a while since I tried NodeBB, but with as many users as https://nodebb.fediverse.observer/dailystats shows, I would expect to see more crossover