There are a couple of key elements to consider.
One is the video data itself. What will happen when a hard drive fails...because they will, you just do not know when?
The storage subsystem is different between the basic NVRs and the servers. The NVRs will mostly be 'JBOD' storage and thus when a disk fails..all the data is lost from that drive.
As you look at servers, they will start offering RAID support, which will keep the data better protected with different degrees as you go from RAID1, 10 up to RAID 5 or 6.
Another consideration is the VMS itself. The NVR with embedded VMS may not have all the features that you want and/or need, which will push you up to a VMS with those features and the system type that is needed to run that VMS. Camera support is also an important one here. The ONVIF spec helps in this regard.
The ability to 'upgrade' the hardware can be a consideration as the customer adds more devices and the system demands more compute and memory resources. This is easier to do on a server.