I checked out nodebb using the nodebb community forums and personally I wasn't too crazy about it. All the forums were a weird scattered mess and the thread listings were just that usual kind of modern flat, featureless look that everything's using nowadays... all that's fine I suppose since I imagine you can just theme it and make it look a bit more traditional and less scattered.
The bigger issue is that pressing back in my browser didn't seem to work and I don't know if that's a bug or an intentional feature or not but I could see that being really annoying if I can't simply jump back to the forums list with the back button. This got annoying to the point that I really didn't mess with it much further then browsing around a bit. Ther emight be some other annoyances, I don't know.
I don't really know how either forum software is from the maintenance/administration end of things so if there are any major benefits to nodebb for that then I guess that part's important. I'd just hope it's more than something to make it look more "modern" or whatever. Where modern seems to go a bit more form over function and more about trying to look flashy with transitions and dynamic this or that where it's not really necessary.
I think the community would like to hear about the benefits of moving to nodebb over continuing with phpbb so they can have some input. I imagine there haven't been any real responses to this thread since people simply don't even know what to compare.