Well, Arlandabanan only runs from Rosersberg (where it spurs off the main line). From Sthlm C to there, Arlanda express has to go on the same tracks as everything else, including local commuter trains, freight trains, and whatnot. 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.