Not very robust, perhaps prone to problems, not rock solid dependable
IMO, SPOT 2 is not a replacement for a PLB and I would not bet my life on one. For distress alerting I carry a PLB. OTOH, it is far better than nothing and SPOT has enough rescues to their credit for them to be a credible, if not optimum, alternative. SPOT can be a very useful tool and you could do worse for distress alerting if you use it with full knowledge of its quirks and drawbacks.