From what I see quickly on their page, there’s nothing at all stopping you from using it with Nethserver. They’re a DNS server, so just point your DNS settings at them. Would anything else be needed?
For me the pros are able to use the Private DNS feature on my Android phone while outside of my local network while still having visibility over the analytics similar to Pi-Hole
While I like Pi-Hole NextDNS has nifty features for adding block lists and keeping them well updated
I did try AdGuard home as well but found NextDNS to be good as an all round DNS solution
I have set the DNS in Nethserver to point to NextDNS IPv4 DNS servers
With a more integrated solution you can view individual devices as it doesn’t currently forward those from the router
For examply: I can use the Private DNS feature within Android and change the Private DNS to my--phone-xxxx.dns.nextdns.io and then the admin portal sees that as an individual device (the xxxx.dns.nextdns.io being a unique DNS domainfor my account)