It's not the cancellation itself (well, not entirely...): it's that UX almost never fails to handle it poorly, communicate incompletely, and generally act like they really don't care. UA, for their part, tends to pretend like UX is some sort of unrelated entity out there that it wants nothing to do with.

There seems to be some sort of contentious relationship between UA and its UX operators that is unique to them. It often feels very disjointed and borderline hostile...quite different from US, DL, and AA.

I've actually had mostly okay experiences when long-haul UA flights have been delayed/canceled. Reasonably competent staff, decent rebooking options, etc. It's UX that is the biggest problem for me. Especially in their northeast locations and, to a lesser extent, ORD.