The pinger is, of course, externally mounted so that its batteries can be changed without having to open up the recorder itself.
That is one of the advantages of course.
Ultimately, whatever mount used has to allow the pinger/ULD to couple acoustic energy effectively into the water.
I suppose that the CVR/FDR recorder manufacturers have tested the mounting technique under dynamic conditions, however the use of what visually appears to be 1/4 inch or 5-6 mm Allen head screws seems weak should the mount receives a blow in the direction of the pinger axis.
I have sheared plenty of 1/4 inch hardware in my life, and going up at least one hardware size would significantly improve strength of the mounting.
One of the AF447 pingers was knocked off its mounts. Neither was detected. That was not a very high energy impact as crash energies go.
Lessons learned from the AF447 search
What we discovered is that you are dealing with a lot of uncertain information in such a search operation. You cannot count on the pingers: after a crash it is always unclear if they are going to work. They towed the pinger locators right over the wreck shortly after the crash, and they didn’t hear anything. When the black boxes were found, only one pinger was still attached. The other one was never found.