Airstart reliability

A friend of mine recently was involved in an outlanding accident. It
seems as though having an engine on-board would help to avoid
outlandings and therefore outlanding accidents. If there are any
generalities to be made, which are generally more reliable for
airstarts - sustainer systems or self-launch systems? My question
refers to gasoline systems and not electrics. And yes I understand
that no system is completely reliable and one should always have a
safe-landing site available even with an on-board engine. Thanks.
Tom