Though I really haven't worked on it, I feel reletively confident in saying Hammer is perhaps the most emotionally complex piece performed, maybe by a large margin. I also think that the technical problems seem easier than they are because though they don't look like much (well, at least in comparison to some things), there is no 'falling in the fingers'. Like the Brahms pag variations or something; they don't look too horrible, but they are incredibly hard. Liszt and Alkan even more so are very virtuosic, (i've played almost no alkan, just making an educated guess) but aren't hard in the way the brahms is. They require a lot of technique to play, but once you have the technique you are home free. Not really so in the Brahms, and hammer klavier is like this. It will never be easy.
Anyway i think the true difficulty lies more in the emotional complexity of the piece than the technical issues.
[geez i rambled more randomly than usual even]