They might not be safe..... all the OHJ does is move the point of possible aircraft / aircraft conflict. Ever been in the situation where you've called "Overhead" and begun the descent only to hear another aircraft also call "Overhead" immediately afterwards? From personal experience, it quite rightly makes your hair stand on end. Is the second joining aircraft above, or below? Is he/she on the correct altimeter setting yet? Are they aware of your presence?
ShyTorque, I agree that no system (even the US 45 degree join) is completely "bullet proof". But broadly speaking if all a/c are joining from 2,000 ft in the correct manner the first a/c to call "Dead Side Descending" will be ahead of (and theoretically lower than) a following a/c etc.
I don't see the point of calling "Overhead" and then subsequently calling "Dead Side Descending" as is, I believe, shown on the CAA pic of an OHJ. You have already called for joining information (not "instructions" assuming you are talking to A/G or AFISO) with, ideally, some indication as to position for the benefit of other a/c. To my mind an overhead call can create more confusion.
That said the main thing that concerns me are those who descend on the dead side in a straight line (often parallel to the runway) rather than in a descending turn which has the benefit of a) clearing airspace below and b)a good view of the runway to look for a/c taking off and/or going around which may affect where one can intelligently fit into the traffic pattern.