Ned Simmons wrote:

I can see where a bearing with more accurate geometry might be
stiffer as a result of better stress distribution...
Yes.

but I'd expect that
would make it more robust, not less.
It will last longer in proper service. It also is more susceptible to
overloading from thermal growth, misalignment, etc. If you're going to use
Class 9, everything in the setup had better be perfect. If it is, it will
outlast a lesser-quality bearing.

This doesn't make sense to me. If a high ABEC class bearing will
outlast a lower class bearing under ideal conditions, what's the
mechanism that will cause it to fail sooner in a less than ideal
installation?

Well, he didn't say fail catastrophically. It could be
that it degrades in performance and doesn't meet class
9 specs any longer.