The downsides of working at UCLA as a graduate-student instructor are similar to the downsides of that job at any large, research-oriented institution. You will be paid very poorly for the work you do under the guise of being "trained" in your career---even if you've been in "training" for years. The student health insurance here is not awful, but it's also not great, and you are not going to get vacation or sick days. There is a mandatory retirement plan. Mentorship from faculty members can be woefully hard to come by, particularly when it comes to teaching, which is not valued as it ought to be. The campus is so large and has such a long legacy as a commuter campus that meeting all the brilliant and fascinating people here can be exceptionally difficult; the size of the university can also be a big problem administratively--the bureaucracy is often poorly coordinated to the point of being nonsensical, and getting hold of the right person to fix things can take ages. Although your schedule as a grad student is fairly flexible, you can expect to work long hours: your job is always with you, there's always more to do, and if you teach competently, it usually will take more than the 20 hours/week for which you're (badly) paid. Generally speaking, your job-performance reviews as an instructor will come only from your students, which means that you are inevitably subject to revenge-motivated poor evaluations if you try to hold them to reasonable standards of performance. This also means that teaching awards can be a matter of popularity with undergraduates--and hard to come by if you are teaching a difficult class or expect your students to work hard.