The vet career is very well done. You greet a patient and take then to the diagnosis table and examine various body parts (like fur/skin, their mouth, their eyes, ears, etc.) and it tells you what is wrong with the pet (much better than the human doctor which forces you to remember what is each disease is and what treats it) and how to treat it. Since the code in there for the vet career, they can easily update it for the Human Doctor to work the same. I haven't played the scientist much, but I do like the cop career. Guess because I like mysteries.
In regards to the open worlds in Sims 3, I found a person on a well known CC site who has fixed the routing issues with many of the worlds (both worlds that came from the game itself and store worlds) and I play with her fixed worlds in my games. Yes, even Isla Paradiso which everyone says is a huge mess in terms of routing, etc. So, fixes to any Sims version can be done IF there was an editor (like when you write and publish a book) and someone was willing to recode the game. Twillian (NRAAS) fixed many of the issues with his game mods for Sims 3. Sims 2 for me was fixed by Aspyr who ported the games to the Mac OS.