In my JLPT textbook, it defines ～もさることながら as saying that the speaker feels one thing is true, and in addition another thing is true.
One example they give which makes sense to me is:
美{うつく}しさもさることながら、彼女{かのじょ}の魅力{みりょく}は何{なん}と言{い}ってもその人柄{ひとがら}にある。
Here the speaker is saying that while the woman is beautiful, it goes without saying her personality is part of her charm. Note I'm not trying to make a perfect translation, just focusing on the essence of ～もさることながら.
But there is this sentence in a test question in the book:
舞台{ぶたい}のセットもさることながら、俳優陣{はいゆうじん}もこの上{うえ}なく豪華{ごうか}だ。
I'm a little fuzzy on what this is trying to say, partly because 舞台{ぶたい}のセット is just a neutral thing. There's nothing inherently good or bad about it. Unlike, for example, 美{うつく}しさ, which is presumeably a good thing.
To me, the sentence seems to be saying something like, "While it's true that's a stage set, the acting group is no more superior." I might be wrong about what 上{うえ}なく豪華{ごうか}だ means too, but partly that's because my confusion over ～もさることながら is giving me little context to build on.
The point is, I don't see how a stage set can be said to be true or not true in any way that one can attach ～もさることながら to it.
So I'm probably wroing about what ～もさることながら does. Does it imply a judgement? Is the sentence in the question saying that the stage set is good?
