The fallout from the recent massive CSS-related changes in the core suggests otherwise. All we received was a bunch of bug reports about issues we already knew about.
I fully expect other large-scale changes to the core to follow the same pattern -- things simply don't get into the core code without us being fully aware of the impact.
That's not to say, of course, that all the code is free of bugs (that would be ridiculous) or that we don't value the reports we do get. It does indicate, however, that the amount of testing we do before things get into the core results in far more robust code than you might expect.