Examples I have seen are
例文を作りにくいです。
なぜ大人になると友達を作りにくいのか?
I thought にくい transforms verbs into a イ形容詞.
にくい can transform both verbs and verb phrases into difficult to ....
Here を is used because 友達 is an object of the predicate 作る, and にくい makes the whole verb phrases into adjectives.
What you had in mind were probably
which are also valid (also が instead of は is possible). Here は(が) is used because they are subjects of the sentences.
There is not much difference. The former describes a property of action (making sentences or friends) and the latter a property of sentences or friends in terms of making.
Additional comment
Usages of は/を/が above are not totally interchangeable, but it should require another question.