The Chromecast does look nice though generally, albeit rather odd that it doesn't have DLNA/Airplay support generally (although I guess by adding the Plex app you could get something like that?). But I guess that's not quite what they had in mind for it.
Yes, Chromecast is a bit different in its intent - it's more of a "media player
extension" to your existing smart device.
Plex on Chromecast still requires Plex Media Server running on
something. If PMS is running on the NAS device it doesn't add to the equipment count, but PMS on NAS can't always transcode, so that limits the benefit to a degree.
The only tricky part with DLNA is the variance in support between servers and clients. DLNA defines the protocols for exposing and transferring the media, but it's still up to the server what it wants to show, and the renderer wants to render.
External subtitles (separate files like .srt and .ssa) are especially tricky as not all servers will send the subtitle data along with the requested video.
And of course not all DLNA servers will transcode, so the format of your media still matters a lot.