As of the time of this writing, both Guardians of the Galaxy: Cosmic Rewind and Tron Lightcycle / Run utilize Walt Disney World Resort's® virtual queue system in favor of a standard, slower moving line. The EPCOT® attraction gives guests an estimated, minute-based wait time in the My Disney Experience app, while Magic Kingdom® Park's does not. In both cases, the My Disney Experience app will send guests with a virtual queue reservation an notification when it is time to board.
Both attractions highly encourage guests to show up within their hour-long boarding window, and those who miss that window—especially a significant amount of time—are not guaranteed to ride.
That said, conflicts with dining reservations, Lightning Lane selections and real-life complications can arise. In most cases, guests who have been late to Guardians of the Galaxy: Cosmic Rewind have been allowed to ride after scanning their park ticket at the attraction's entrance. Though this may not be the case for every day, especially during the parks' busy seasons. the large capacity the attraction is able to see at once allows it to progress rapidly through boarding groups. This often leaves adequate time for those who are late to be able to ride.
However, Tron Lightcycle / Run does not maintain the same operating capacity, and guests have been turned away from riding for being late to their reservation. The attraction's limited capacity means too many guests missing their reservation time could prevent guests later in the day from riding, so guests are encouraged to arrive during their reservation window. Those who miss their window by a significant time may not be allowed to ride the attraction.
Guest experiences representatives—often wearing blue—are often near the attraction entrances to help when extenuating circumstances do arise. So if an emergency happens and you miss your boarding window, there's no need to worry, as they are on-site to ensure you still get your opportunity to experience the attraction.