publications - Citing (or not) a flawed or incomprehensible paper or preprint

Is there a good practice of citing (or not) a paper or preprint that you consider flawed (or at best - totally incomprehensible)?

Once I had a problem of that sort. I wrote a paper on a topic, which was not very popular. Even if I was not using other's results directly, I wanted to cite a few papers solving very similar issues.

Then I had a dilemma if to cite a preprint tackling the same problem, using methods I don't understand (with a feeling that it is incomplete, flawed or just extremely badly written).

Ii that case it is better to:

• simply drop it,



• cite but make it explicit that you are just mentioning it, not using their results,


• or cite making it explicit that you have serious doubts on its content?

EDIT:

By a preprint I understand sth which is archived on arXiv or sth similar.