I agree that this is a huge problem with UA and I also check all my reservations at least once a week.
During my check last weekend, I discovered that an upcoming reservation LGA-ORD-BIS RT had magically dropped the ORD-BIS-ORD legs.
Imagine my surprise when I called and learned that UA has dropped that route and didn't re-route me through DEN.

No email. No call.

I feel sorry for those less-experienced travelers who learn of these kinds of changes when they check in for their flights.