I've only one experience with EU261 and it was rejected. I couldn't be bothered to challenge. I feel "auto reject" isn't that uncommon.
Rather than perhaps auto pay, it is clear the airlines should be pro actively issuing literature when they are to blame. The recent threads on the 48 hours delay from LAS, this was not issued.
Perhaps those stuck for 48 hours might have had more fun on the Strip and been less annoyed if they'd known they had a sure fire 600 Euros to play with