Hi; I think the structure is clear anyway (you don't omit "also"). I also add that in my language (Italian) but also in English, that sentence would reach even more effect with the use of the "dash" ( [ – ] or [ -- ] ) instead of the comma. Dashes insert a comment within the sentence, or introduce a comment at the end, and makes this comment more "evident". (Russian and Polish use it in a slightly different way) In this case, it creates a greater contrast with the first part; so I think using the dash in the place of "but" is appropriate. --- "Not only is her art experimental -- it is also controversial." Of course, you might not like this "suspence effect", according to the context: if you stress so much on "controversial", perhaps you would also like to explain why: --- "Not only is her art experimental -- it is also controversial. In fact, in her last works (...) " Otherwise just let the comma.

I'm not a teacher or grammarian. I think your second sentence, "Not only is her art experimental, it is also controversial," is fine, and better than the first version. (Whether or not it parses correctly grammatically, there is an unwritten universal practice of leaving out words if the listener can effortless guess that they are there). Michael Abela's suggestion is fine, too. The problem I have with it is logical. "Experimental" and "controversial" aren't opposing concepts. It is not surprising that "experimental art" would be "controversial." In fact, you would expect it to be. Therefore, I don't see this as a situation where "not only"/"but also" applies. Let me put it another way: your phrase would be appropriate in the following hypothetical situation. Suppose you had an exhibition of "experimental art," and six of the artists were showing pictures that were experimental but were bland, harmless, and didn't bother anybody, while the seventh artist's pictures had some kind of strong political message that offended people. In this case you might say "her work was not only experimental, but controversial."

I would rather write it this way: Her art is not only experimental, but also controversial.