The big problem with HW2 is that it's just very undocumented. You have to find out a lot of stuff by trial and error, and there isn't much commentaries in files to help you out with this. One significant thing is also that HW2 modding is basically dead, or dying for the most part. We are only a handful to still be actively modding it, literally.
To be honest, the UI is most of the time the least of the modders' worries when making a new mod. Getting assets ingame, creating effects, balancing stuff and creating gameplay mechanics are the most painful overall.
However if you look closely, HW2 is very moddable and powerful for a 7 years-old engine. Nearly all aspects of the game can be altered, ranging from effects, ships, AI, maps or yes, the UI itself. It is even superior to FSO in term of optimization and consistency of framerate, with similar assets, at least according to those who tested my mod.
One of the main problem is that it's a RTS, and hence you need to completely rethink the gameplay mechanics, the balance and the gametypes to have something that doesn't feel like a simple swap of models over the retail game. This aspect is much more sensible than in a game like FS, where having new ships and new missions is more than enough to enjoy a new experience - especially since single-player campaign scripting is virtually nonexistant in HW2 modding. Not that it's impossible, only that noone has attempted it yet, and because until very recently, we didn't have any properly decompiled campaign scripts to base ourself on.
The second main problem is that the engine isn't going anywhere now, whereas FSO has still a bright future before it. The new shaders are a substantial upgrade over the engine, but you often encounter hardcoded barriers that you have to find your way around when trying to implement new features - for example, slash beams, that I still have no idea how to implement in HW2.
Keep also in mind that this mod is still a one-man-made Total Conversion. You can guess how a huge amount of work it is. And feature creeping and real life are eating most of my time.