Hard to get classes?

I keep hearing that it’s hard to get classes at some of the UC’s. A guidance counselor even advised my kid not to apply to Berkeley because of this and other problems related to a tight budget. Even though the tour guides say otherwise, I know there must be some truth behind the GC’s advice–on UCSC’s web site recently I saw an announcement to students explaining that summer session offered an opportunity to get classes they were shut out of during the regular term.

Any current or recent Berkeley or UCSC students or parents here who can shed light on what this means in a practical way? How widespread is the problem? Does it affect some majors more than others? Is it something that can actually cause you to add an extra semester? or is it just at the level of minor inconvenience?