I've always been curious about P2P. It uses third-party servers, so no doubt it's less secure than someone's personal VPN, but I just couldn't believe that Dahua/Hikvision/etc would offer a free service that relayed everyone's video directly through their servers. The bandwidth costs to do that seemed like they'd be insane. That, plus when I played with Dahau's P2P (back when I ran one of their NVRs), it was quick/responsive. Definitely did not have the lag I'd expect if the video was being relayed through a third-party server.
From what I read today, most P2P (peer-to-peer) services arrange for a direct connection between two devices. Once that connection is made, the two devices exchange data directly between each other, using their own bandwidth... the P2P service is out of the loop at this point. OK... so if P2P services don't have to use their own bandwidth, this is starting to make a little more sense how they can be offered for free...
But how can a P2P service connect a device outside of your home network to a device inside your home network when you don't have port forwarding setup? Most use a technique called UDP hole punching or STUN. You can google those terms for more info.. UDP hole punching has more easier-to-understand explanations IMO. Those techniques work with most routers/firewalls. In the off chance that a router doesn't allow it, then some P2P services will act as a relay, while others will say "connection failed", and tell you to try again.
UDP hole punching sounds like a bad thing, but its how TeamViewer (an app some folks here will use to remotely control another's PC to help them out) and Skype get devices behind firewalls to directly connect without port-forward (or having their servers in the middle of the conversations). I think Skype might actually be on to something else now, but they used this method for a very long time.
My concern with Dahua's P2P service would be around how safe they keep everyone's P2P account info. If that ever gets hacked, then it could be used to connect to other people's cameras/NVRs. But for folks where running their own VPN isn't an option (for whatever reason), I'd think Dahua's P2P service for remote access would still be better than port-forwarding.