Like most things: it depends.
The nVidia ShieldTV is an amazing device. It is amazing in the wrong market for it to look amazing. I don't want to spend six paragraphs justifying the device, so I will suffice to say that the ShieldTV is a device which does far more than expected at a price point such features shouldn't be possible. However, it is marketed as a streaming device, in which category its price is actually a liability that turns people away and more's the pity.
That said, one of the amazing features of the ShieldTV is its ability to do double-duty as a Plex media server. Consider the fact that I've been running some form of a media server (not always Plex-based, though I jumped on the bandwagon pretty early in the Plex game) for close to twenty years. In that time, I've invested close to $5000 into the privilege of having this thing. That's a pretty good breakdown of time versus cost, but it should also highlight why having a device that sells for less than $200 and is capable of filling the same role is downright amazing.
So, the question is: is the ShieldTV good at being a media server? Yes. To an extent.
Qualifiers. The ShieldTV can stream music (particularly tiny files like .mp3) without any real strain. It can stream 480p content, again, without noticeable drain on system resources. It can transcode 1080p content like a champion. So, where's the downside?
1. The ShieldTV does not have the power to transcode more than 2 simultaneous streams. So, if three people try to connect and watch a 1080p movie on their cell phone at the same time, there's going to be a lot of buffering while the device struggles to keep up.
2. The ShieldTV does not have the power to transcode and perform other tasks well at the same time. You can still use the device while it is transcoding streams, but it will be noticed. The system gets a little sluggish and using Plex natively can become problematic.
3. The ShieldTV does not manage large libraries very well. If your library is somewhere north of 500 items, Plex is going to struggle with it on the ShieldTV.
My conclusion, then, is that the ShieldTV is a more than capable device to run a Plex server for the vast majority of users. Think of it this way: you bought a device so that you could easily stream 4k content to your television. The ShieldTV is far and away the best option, so you spent a little extra to make sure you got the best on the market. Along with that industry-leading power and design you get the ability to share your own content with yourself anywhere in the world as long as you have access to the internet. That's a pretty great free extra feature.
On the other hand, if you're like me and you store all the music, audiobooks, comedy acts, movies and television shows for your entire family (parents, siblings) so that they will have access to it anywhere at any time, then the ShieldTV is not the optimal solution. I've spent a bit more so that I have a device that can handle 5 simultaneous transcodes and enormous libraries of thousands or tens of thousands of items.
Is the ShieldTV a good Plex server? Yes, for personal use with relatively small libraries (which should cover more than 90% of all users).