1. "They're not so much lovers as friends."
2. "They're more friends than lovers."
I'm a bit confused in analyzing what "so much" and "more" modify in each sentence. I think "so much" and "more" modify "lovers" and "friends" in each sentence.
Note that (1) can be rephrased "They're not lovers so much as they are friends". I would say that "so much ... as (they are) friends" is adverbial, expressing the extent to which, or way in which, they are (or aren't) lovers.
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Note that (1) can be rephrased "They're not lovers so much as they are friends". I would say that "so much ... as (they are) friends" is adverbial, expressing the extent to which, or way in which, they are (or aren't) lovers. Ditto for "more ... than (they are) lovers". Other analyses may be possible.