Consider the sentences:
The method is less efficient and secure against hackers.
The method is less efficient and less secure against hackers.
Both sentences can work to communicate the same thing, but (1) suffers from a structural ambiguity, whereas (2) does not. (2) is perfectly unambiguous.
The reason (1) is structurally ambiguous is because there are two syntactic structures which might underlie it. The first is the intended one where "less" modifies both "efficient" and "secure." The second is the unintended one where "less" modifies only "efficient." On this disambiguation, the sentence communicates that the method is less efficient and (nonetheless) secure against hackers
In short, if you definitely want to avoid ambiguity, go with (2). But keep in mind that (1), though ambiguous, is likely to be interpreted as intended.