There are actually two different questions about competence:

Competence compared to some objective standard, basically "Can I do that?", which I think often can be answered realistically (at least where "that" is specific enough).

Competence compared to others, or how do my abilities rank compared to others, which may not really be answerable, except very generally and vaguely.

The big problem comes in trying to generalize from specific competencies to "general competence".