ACARS has several component systems; some are common to all airlines and others are unique to each particular carrier. At its simplest form it provides very basic information regarding the status of a flight (takeoff, landing, gate departures and arrivals) and in-flight weather information (making each plane in to a temporary weather station).
Some air carriers use ACARS for much more comprehensive functions like monitoring the status of equipment on aircraft in flight (maintenance scheduling), passenger information or cargo requirements. Those carriers are going to be more susceptible to periods of high demand across bandwidth limited VHF frequencies or availability to a satellite uplink.
ACARS has become so useful that it is now hitting some limits because of the high demand for the functionality.