The course offerings are generally constrained, and the courses themselves are often lightweight versions of the university course -- assuming that the subject you want is covered at all. The facilities for continuing education courses are also often less robust.
Take CCA in San Francisco. This is a highly considered arts college, and they offer a small number of introductory extension classes:
Compare to their full course offering:
You'll have to drill down on individual categories to explore the full breadth and depth.
Continuing Education offerings are rarely a substitute for the quality of courses they offer to a matriculating student.
Given how often educational institutions have raised their tuition in recent years, I'd love to see them institute a program where they charge a (possibly large) fee to allow non-matriculating adults to audit a course, without limitation or requirement, and use those fees to subsidize enrolled student tuition.
However, this may be a shift too far in the "trade school" direction for their comfort.