Are you comparing it to KDE as in the software ecosystem / integrated desktop experience? "Fully" would be a misleading way to compare in that case. Someone who lives in a given text editor and needs a pleasant, lightweight host with extra nostalgia built in may really love it. Grass-is-greener holistic consumer software critics probably not so much.

Depends on your standards. CDE is, by modern standards, rather feature poor; if you actually use any significant slice of KDE's features it'll feel very limiting. On the other hand, IME it's also far less buggy than KDE (possibly for similar reasons; your features can't break if they don't exist), and it's stable both in the sense that I don't think I've ever seen it crash, and in the sense that I don't think I've ever seen it noticeably change. Mixed bag, YMMV.

CDE was not great, but KDE still has major UI issues. Was just trying out the new Konsole and the "show menu" setting changes places between the menu <--> context menu, depending on whether it is set or not.

Also the silly new toolbar buttons were really hard to find to hide, shouldn't have to google change such basic stuff. Tells me that no one is paying attention to these details. Iterm2, while not perfect is much better at these things. As is gnome/mate-terminal etc.

There is no font compositor; text is less attractive.

There was no HTML browser for the early life of CDE. NCSA Mosaic was the first, followed by Netscape for the RISC platforms.

The native terminal app, DTTerm, is not tabbed. Such a GUI element would take another decade.

Don't turn on the server components unless they are patched (ToolTalk specifically).

There is a GUI-enabled Korn shell, dtksh, but it is an ancient version the last time I looked. Most scripting sourced library code from a book by Pendergrast (of which I found a copy after lots of searches). I used dtksh for basic operations dialogs in the past, so I had some interest.

It is fast, though.

I used CDE professionally back in the day (a long time ago). It was very good in its day, but I use KDE now, and I would never go back to using CDE. If I were feeling nostalgic for old software I'd rather go play with TOPS-20.