Errors of faulty parallelism in sentences in which “not only” and “but also” help delineate complementary phrases come in three general categories, as shown, explained, and corrected below.
1. This problem not only relates to accessibility but also to completeness, accuracy, and validity of the data.
In a simple sentence employing “not only” and “but also,” a verb that applies to both phrases must precede “not only”: “This problem relates not only to accessibility but also to completeness, accuracy, and validity of the data.” (Otherwise, the assumption is that a verb distinct from the one following “not only” will appear after “not also” in parallel to the first one, as in “This problem not only relates to accessibility but also applies to completeness, accuracy, and validity of the data.”)
2. This step presents not only a technical change, but introduces risks associated with migrating to the cloud.
In this example, parallel verbs should follow the respective setup phrases “not only” and “but also”: “This step not only presents a technical change but also introduces risks associated with migrating to the cloud.” (Note, too, the deletion of the comma and the introduction of also.)
3. In this way, the courts have been central, not only to the preservation of American freedom, but also to its expansion.
In “not only . . . but also” constructions, a comma is often inserted before “but also” (or before but alone when also is not included, as in the example above), but the punctuation mark is unnecessary because what follows it is not an independent clause or a parenthetical phrase. Here, the first comma is correct, the second one (assuming the third is omitted) is defensible for emphasis but is extraneous, and the third is a mistake, as explained in the first sentence in this discussion: “In this way, the courts have been central not only to the preservation of American freedom but also to its expansion.”
Furthermore, the appearance of the second and third commas together is a double error; the inclusion of this pair of punctuation marks erroneously implies that what is contained within is parenthetical. (To test for the validity of the punctuation, view the sentence without the intervening phrase: “In this way, the courts have been central but also to its expansion” is ungrammatical, so the commas are incorrect.)