That's why it's as vulnerable to disruptions as everything else. In fact, this is one of the reasons I prefer the bus. If it breaks down along the way, you can always hail a cab. In the train, you're stuck.
The problem with car/bus on Stockholm city-ARN route is, the sector between Upplands Väsby and Arlandastad intersections.
If there is any car accident along the way on this sector. You get stuck. No other choice.(Actually I saw once a Volvo X70 drove out to the fileld and headed to Gamla Stockholmsvägen near Stora Wäsby. S/He must be desperate.)
That's why I take Gamla Stockholmsvägen from Upplands Väsby when I drive to Arlanda. Just in case.