I have not messed around with chem lights for several years, so perhaps things have changed, but on at least six occasions either I or someone in my party has ripped off the covering from a chem lite and snapped it, only to find - no light. I did carry the more durable kind and I found them to work much better, although I only had limited experience, perhaps three samples.

The ones that work are made specifically for installation on a PFD; they have a large clip for that purpose attached to the tube. I am not sure about brand names, but they did have a USCG approval number.

The problem with the normal variety seemed to be wear and tear on the outside covering; the ones that failed were worn and displayed pinholes.

There are electronic versions of chem lights that I suspect are more reliable, if equipped with good batteries. I have not tried any of them.