This is one of the greatest, little-played works of the last century! I used to have my students translate and interpret the text, which is admittedly difficult.
But the music is a marvelous hybrid, with distant echoes of Gurrelieder and future echoes of Moses und Aron.
But what is amazing for an unfinished work, is that it does not sound unfinished at all. The ending with the single note sung by a single voice, is absolutely, ineffably beautiful.
Such a great work done in by the text, would you say? Or are the problems musical, as can happen with Schoenberg?
The text, for me, offers no roadblocks. But I can see where the general concertgoer might be put off (their loss...).
But the music...it can stand with the best of Schoenberg, which is to say, some of the 20th century's finest. This alone should put it in good standing.
Being new to the work I had wondered about its romantic leanings, wondering if it simply had been nudged that way by Nagano. But your description of it as a hybrid makes sense. Elements of the past finding refuge in the future.