Hi brass,
I don’t own one of the thin “membrane type” protectors so I can’t share any of my own personal experience (outside of using them in the past with no issues) but every one of them I’ve seen has had the same type of mixed feedback that you are mentioning that varies from “works great” to " completely failed" although the second is usually a much smaller percentage. I suspect that some of them are from not following care instructions but there always seem to be others that just simply failed.
There are also different types of technologies and materials used to make the semi-permeable membranes (see the last part of post #2 here) and the type and thickness of the fabric that is bonded to the membrane will also play a role in the cost and performance but the different manufacturers don’t really supply the technical details about how their membranes are made so I’ve never been able to identify whether one type is clearly superior to any of the others.
My own mattress protector is the Naturaprotect Deluxe which also has a membrane but also includes 10oz/yd of wool and you can see my comments about it here. It has seen several “accidents” (sickness and spills) but none of them have penetrated through to the mattress. I personally doubt there would have been an issue even without the membrane and if I had to do it over again I’d probably choose a wool/cotton protector such as the Dormeir that was a little thinner and doesn’t have a membrane which can reduce airflow and lead to heat retention issues for some people.
Hopefully others that see your topic will also share their experiences with the protector they own (and a forum search on specific brands may bring up some feedback as well about some of them) although it’s quite likely that if you were to receive a larger number of comments about any one of them that the feedback will be just as mixed as the online reviews.
Phoenix