If there is supposed to be some kind of testing & certification of the low berths I have not seen anything in CT materials to spell that out clearly.
As far as I am concerned, the low berth mortality rate reflects old poorly-maintained equipment on old poorly-maintained ships (the kind of 40+ year old merchants and converted scouts that PCs can afford to buy passage on or buy themselves).
Add in crew with little or no experience in actually operating the low berths (even PCs with Med skill won't have had much if any formal training in cryo-techniques), and you have a recipe for "death by misadventure"*.
Aboard military vessels, with high maintenance & training standards, and with medical personnel specially trained in cryo-techniques, and with subjects in good-excellent physical condition and with well-documented medical conditions and records, the "frozen watch" is a process with an acceptably-low failure risk.
Medium-large passenger liners also have highly-maintained equipment, highly-trained medical personnel & attendants, and demand in-depth medical records on passengers requesting "cold-sleep passage".
* A death by misadventure, as recorded by coroners and on death certificates and associated documents, is one that is primarily attributed to an accident that occurred due to a dangerous risk that was taken voluntarily.