I remember a comment someone at UDEL made when we'd been working with v6 for a few years, starting in 1976. "v6 is fast as hell but it has its bugs."
It was a big deal, at least in 1976, to get through a week without a crash of some sort. That's why we had all those pre-fsck programs for looking for problems: icheck and ncheck existed for a reason.
The -11 also had its corners, e.g. alignment traps to snag the unwary, and hardware that, in several cases, did not completely honor the unibus spec. If the hardware has limits, or corners, and your kernel crashes because of it, is it a bug that you did not accommodate the undocumented hardware design? Given that you can't really go at your machine with a soldering iron (well, not always; we all did some of that back then) it's arguably your bug.
Unix code was great, far better than a lot of what was out there, certainly on the -11, but to say any of it shipped bug free certainly does not match my recollection.