I have to agree, at least on the "modern" part. I compiled it from sources last summer to run on a retro-style Linux LFS I was putting together for fun. It was fun and nostalgic to see CDE running again, but there were parts of the build that didn't work at all (eg. help viewers and documentation), and support for Xft antialiasing was still pretty spotty.
It is still receiving patches AFAIK, and maybe these issues are fixed by now. But I also couldn't help feel it was probably a ticking time bomb regarding security fixes etc.
I also installed it from source somewhat recently and the lack of help viewers and documentation makes me sad. You have to learn by exploration, which is not easy in a system with so many parts.
Yes the config files are described in manual pages though, remember that most of the users that worked on this were working on green-screened text terminals only a few years earlier. So this wasn't a big leap for them and more of a natural transition.
But I also remember it took me some time to figure it out. Especially because it required logging in and out to test it, and if there was a big mistake the login would fail and I'd have to jump into a text shell to fix it :P
It is still receiving patches AFAIK, and maybe these issues are fixed by now. But I also couldn't help feel it was probably a ticking time bomb regarding security fixes etc.