It's not really about whether it's 1080p, 720p, 480p, 4K HD or anything else - it's all about the bit rate and whether the infrastructure (whether wired or Wi-Fi) has the capacity to deliver it in a timely enough fashion so sustain playback without buffering.
The content author decides on the bit rate when they create content (or transcode it.) The video format is the very roughest of "rough estimate" indicators. For example. I can stream 1080p videos down my 48mbps Internet pipe and across a 72mbps Wi-Fi links without a problem - but that's because the bit rate is "only" 1.5mbps or so (and therefore quality is lower, despite being "HD.") Whereas my Blu-Ray rips are much higher, typically of the order of >25mbps. I've got "mere" 576P DVD's with bit rates of the order 6-8mbps.
If you really want to get into the data networking "numbers game" then it's best to delve into the bit rates of the media, the video format is not particularly indicative. A tool like MediaInfo (freebie) is good for this.
In a Wi-Fi cell, "only one thing at a time can transmit" - the more things there are, the more data they want to transmit, the more competition (it's anything but "fair") there is for some "air time." Through in some poor/variable signalling conditions and it can quickly become overloaded.
The same is also true for each lobe of a (cabled) ethernet infrastructure, but ethernet is generally faster and more reliable so it takes a bit more load - especially in lightly loaded SOHO networks - before it becomes a challenge to ethernet, especially if the ethenet is mostly 1000mbps (gigabit.) Thusly, it's a good idea to get as much as possible onto wires, especially for things that don't move, thereby leaving as much air time as possible for the remaining Wi-Fi clients. Of course, domestic harmony and DIY skills sometimes dicate where we can run in UTP cables.
Many of us at AVF have long argued that real time trans-coding is best avoided if at all possibly, then you don't need a NAS with the super powerful hardware required to do it. For just basic storing and retrieval of files, almost anything will do for the SOHO use case. Save the money or spend it on additional storage instead. If you have devices that cannot read a particular file "native" then do a one time offline transcode with something like HandBrake (also free.) Even if that means storing multiple copies, say one full fat version with with all the multi-channel sound a latest video format for the "big" TV where such things matter and one more "universal" version with just basic 2 channel audio and a max compatibility video format such as mpeg2 or H264 (mp4) which is increasingly widely supported these days.
And just in case you haven't realised it, Plex is by not means required to do media streaming. I don't use it. But there are plenty of people that do for various additional features it provides.
