AY reliability is better than average now. The problem is twofold: they operate MD-11s which had reliability and consequent dispatch problems a few years ago (seems better now) and they have a small fleet so hard to get a substitute plane if there is a disruption. This is helped a lot by Oneworld (they can reroute JFK-LHR-HEL fairly easily). From JFK this really is not a significant factor as you won't be stranded.