We still have never heard of something being totally impossible to replicate, just impractical...
The very concept of "reusing an accident" flies in the face of reason. Accidents are accidental and for that reason already extremely difficult to reproduce. Could you really see Starfleet waiting for the perfect storm at Nervala IV and then start probing the atmosphere with twin transporter beams in the hope that the "phase differentials" will again be an exact match for one of them? They'd end up blowing up six transporter platforms for one poorly duplicated crate of dilithium - and they wouldn't get any better at it with time, as the circumstances would be constantly changing.
As for age reversal in "Unnatural Selection", it hardly amounted to that as such. It was reversal of a debilitating illness that had the same symptoms as advanced age. As a cure for arthritis, it seemed to work fine, but there's no indication it rejuvenated Pulaski's cells or anything like that - indeed, there's no indication that the illness aged the cells in the first place. It just gave Pulaski instant arthritis.