I'm also a big fan of pfSense, joining your community
I don't really know Nethserver yet except features and quick look at interface (though demo link).
From my perspective, there is no real competition between pfSense and Nethserver as they do not target same scope.
pfSense intend to act as firewall even if some packages help to extend this scope with HTTP proxy, anti-virus, and stuff like this. One can't look at this as UTM or all-in-one server. Account management is very basic and doesn't aim at describing company users.
In term of infrastructure design, if your need is to deploy FW handling DMZ, VLAN, VPN accesses etc... this is most of the time because these networks host more than clients but also servers and services. With such infrastructure already in place, would UTM fit?
I might be wrong but the way I currently perceive Nethserver is to act as the unique piece of infrastructure on your network, bringing almost all services in one single box. For sure it has to embed firewalling features but also additional services.
To me, the real question, not related to pfSense or Nethserver, is to decide where and where UTM fits or not. Where is the limit of such design? What are pros & cons that would lead to "all-in-one" deployment?
I already have my own opinion, not based on Nethserver yet (later for sure) but on Zentyal which is quite similar to Nethserver, and definitely I would not compare one-to-one pure FW with all-in-one.
To be honest, I'm not a big fan of packages installed on pfSense. It makes sense some times but most of the time, if you feel you should deploy lot of packages on this firewall, then you're ready for the all-in-one design