Curious to know if people are running NTOPNG on their OPNsense firewall or do they offload the Netflow data to another system, where NTOPNG is running?
I tried both, the graphs you get when using netflow into ntopng are a bit minimalistic since you don't use protocol detection like with pcap style ntopng.
If you really want to use it in production for longer, configure a span/TAP port on your switch and put a passive system there with ntopng.