Even if you can evaluate how strong a resident is through your lens, it's incredibly hard to generalize anything when your sample size is...what, 9 months of rotations? How many residents did you work closely with? 20? How many DOs? IMGs? USMDs? But you can already sense the competency of each, globally?
I've had a little more experience than you, and I still can't say definitively which group is better...particularly at WHAT-- there's plenty of aspects to being a doctor besides understanding concepts. I don't even know how you even know who's who. Sure, an FMG is pretty easy to pick out most of the time, and some institutions put DO at the end of the resident's name. But a lot of them don't. And USIMGs? Unless you ask, you wont know where they're from...and I often didn't ask, because who cares?
My general experience was that FMGs were VERY paternal-- as in did not like the patient making ANY choice that deviated from their own. And more brusque in their bedside manner. And the USIMGs I've met trended toward party-types-- few seemed to go home and study. But very few that I met seemed incompetent, and to my sensibility no more often than USMDs. And I have the egotism to label myself an above average medical student-- all my residents have consistently demonstrated funds of knowledge much greater than my own.
In terms of medical knowledge though, no USMD or USIMG even rivaled the cream of the crop FMG's I've worked with, some of whom practiced medicine as attendings in their home countries, a few even being prominent professors. Those guys have a fund of knowledge that is utterly jaw-dropping at an intern or 2nd year level and are capable of working almost independently from the jump. Citing names of studies and debating topics I did not even know were clinical entities at that point. That to me is impressive and intimidating during 3rd year. Though I hardly expect every FMG I meet to be a walking Harrisons. Because I know that my n= like 4 and that's not enough to form any global opinion.
It is always surprising to me when scientific minds make sweeping generalizations like this OUTSIDE of academia, but then when they sit down and read a study, they'll be very quick to point out flaws in design, randomness of the distribution, and sample size. And then turn around and say stuff like "I dunno, I've seen a lot of complications with X, so I like to prescribe Y." Uhhh...you did NOT just carry out your own impromptu study and skip the first or second line agent for all your patients because of your PERSONAL, relatively tiny level of experience with that agent? But I've seen it, dozens of times. And no, I do not know where those guys went to med school.