No expert, but well-handled, modern, diesel/electric subs have embarrassed US Carrier Groups in the past.  I doubt the NORK subs and skippers are up to the task, but it is a reality that very quiet diesel/electric subs are problematic--and potentially deadly.

It is true that given enough assets, such advanced subs can usually be detected, but the right assets, in adequate quantity, and in the right place, are seldom available.

The NORK subs are not technically advanced.  Still, something of which to be aware. 

US Carrier Groups generally rely on being rather far out to sea, and relative to the diesel/electric subs, fast-moving/course-changing for defense, in addition to their inherent ASW assets.

Most modern diesel/electric subs are considered short-range vessels, and suitable for coastal defense.  Very few are considered to be long-range vessels because they must spend so much time surfaced, or have their schnorkels available for detection.  German subs operating with schnorkels were often detected during WW II, and so brought under attack  The means of detection nowadays is no doubt better, but whether it has been deployed, and trained-up is another question.