I'd say just the opposite, Plex hands down. The Plex interface is superior, depending upon the platform. Has many more displays options. It runs on a ton of platforms. I run Plex servers on my Mac and Western Digital NAS. Clients I use include my iMac, Apple TV, iPhone, iPad. You can access the server remotely.
There are also auxiliary programs that work with Plex. PlexPY, for example, gives you library usage statistics.
There are LOADS of bugs, some ruining playback completely,
It is true that Plex bug fixes are random, depending upon what the developers seem to think is important. Only help you get is from the forums. But almost all of the of the issues that are important to me have gone away, other than that annoying "your connection is not fast enough", even though the developer HUD on my apple TV shows 80 Mbps.
the client hasn't seen much meaningful updates since its release almost 3 years ago.
Not my experience. I've had lots of Apple TV client updates, the latest one being rather nice.
I've tried to use Infuse Pro on my AppleTV many times since it came out and had too many problems. Tried it just now and again the "unable to connect" error, even though the Plex DNLA server it connects to is running and delivering several other streams with no problem. The primary media I stream are MakeMkv full video rips of Blu-Rays with the DTS-MA soundtrack, so maybe my experience is unusual.
It is true that Plex is more complex, as it has a lot of options. It also requires horsepower on the server, particularly if you are running multiple 1080p or higher streams. Infuse is much simpler, if you can get it to work.