UA seems to have more flight cancellations than anyone. However, when the flight is cancelled, most carriers including UA can, and should, accommodate you on other carriers services with whom they have a ticketing agreement. Sometimes that involves an argument with the airline agent because they don't even know their own policies. They often feel that they can't use another carriers services. Unfortunately, you have to be knowledgeable and know your rights. Here's a quote from UA "contract of carriage".
Delay, Misconnection or Cancellation
a)
When a Passenger’s ticket is affected because of a Schedule Irregularity caused by UA, UA will take the following measures:
(i
) Transport the Passenger on its own flights, subject to availability, to the Destination, next Stopover point, or transfer point shown on its portion of the Ticket, without Stopover in the same class of service, at no additional cost to the Passenger; or
(i
i) At the Passenger’s request, provided that the tariff covering the original transportation permits routing via the carrier which will transport the Passenger, UA will re-accommodate the Passenger in the same class of service on the next available flight on another carrier, or combination of carriers, if the length of the delay to the Passenger’s destination exceeds two hours
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