Oh boy are you wrong on that front.
You are constantly evaluated as a resident. By attendings, your senior, your fellow, even from time to time nurses and pharmacists will be asked to eval you. And then naturally you get evaluated by your program director twice a year.
If your evals are not up to par, I promise you hear about it. And it's a lot harder to argue your competence without a shelf exam to back you up.
Residency and MS3 evals each suck in their own separate ways, and a few common ways as well.
Both sets of evals suck because they're the most random performance assessments you will ever get in your life. What other job assesses you monthly/weekly, based on the experience of multiple bosses who work with you only for a few days each? I've had more than 50 separate attendings in a single rotation, and even on an average rotation I'll always have at least 4 separate people evaluating me. You can't adapt to a bosses quirks over the course of 14 hours or even 14 days, its just random chance whether your personality matches the quirks he/she already has.
Resident evals also suck because they are never, ever positive. There are no grades, raises, or promotions in residency, so everything is basically pass/fail. That's good, because it somewhat mitigates the randomness of them and lets you focus on the comments more as a tool for self improvement than as a career destroying judgement. However the attendings known that as well as you do, and therefore the common experience for most of the residents I've met is that their feedback is either bluntly negative, or more commonly a **** sandwich of good and bad. There's nothing quite like finishing another month of 80 (really 100) hour work weeks and having your boss send you a long list of your inadequacies to thank you.
MS3 evals are at least occasionally glowing, however they suck because they're almost completely random, much more so than resident evals, and yet the grades you receive can determine the course of your career. As most MS3s don't have a real job to do, you're graded on how much you entertain your attendings, how believable your fake smile is, the crispness of your fake patient presentations, and most of all how your attending/resident feels the day they write the eval. At the end of three years of residency I feel like I have no more than the weakest grasp on what my medical students can do, and I would question the insight of anyone who thinks they can do better. Isn't this the best possible way to figure out who gets to be an ENT and who doesn't?
All in all I would say that, while all our lives suck, MS3s have is worse. At least when I'm evaluated as a resident I an feel reasonably confident that someone's pissy mood that day won't derail my career.