As with anything, there are drawbacks to everything.
My wife and I took ASL courses (not for sailing reasons, just out of general interest). So, technically, we can communicate just fine with sign language. Except when I have one hand on the clew of a sail, the other trying to keep the rest of the sail on
deck, and I need her to ease the sheet... How do I sign that?
Radios also fail, of course. We all know that.
As for
radio inside versus outside the helmet, I would opt for outside. Why? Because if something hits the helmet, the last thing I want is a hard box inside (the radio) being driven into my skull. Might as well not have the helmet at all in that case -- but that's my choice. An argument could be made that the
radio box on the outside can catch on lines and stuff.
One thing that also might sway the decision a bit is to look at
racing teams, the professionals. Do they use radios? Yes they do, so obviously they think the drawbacks of possible radio failure is far overshadowed by the ability to communicate complex and clear instructions.
You pays your
money, you takes your chances...