To answer your "what kind of problems" question, first I'd like to mention that the best way to know what problems a patch/expansion/stuffpack gives is the official forums, as that is the most "help-me-please" spammed The Sims 3 place ever. Therapists should make money with it. That said, let me present you some of the problems I experienced:
I couldn't play TS3 for 3-4 months, because I did not know that what was crashing my game so much was lack of RAM; the minimum requirements were upgraded. "Order Omni Plant Seeds" stops working. Vampires will appear everywhere, and even though that doesn't bother me so much, it "ruins the life" of other (dramatic) people. If you hate having your sim be a Celebrity (scandals = loss of friends + negative moodlet, and paparazzi-palooza), too bad, it's inevitable.
On the bright side, I tried solving each and every one of the problems I ever had in the game, and came by many solutions, which now allow me to help people in the forum, shortening their search for a solution.
As for precautions, I think you should do as the little "clear installation" guide (there's one in every patch's notes) says, by backing up your stuff, uninstalling custom content, etc. and your game should go fine.
Not much to say about Barnacle Bay. I have it, but honestly I haven't had time to play it, not even open it, yet. If you like Sunset Valley as much as I do, you won't move to any other neighborhood, so it's probably not worth it for you.