The PMR handsets (446MHz) have 8 channels allocated to them. In addition, the sets use CTCSS subtones, which allows a certain degree of privacy, or more accurately, you can set them to no hear others - you still share the channel though.
CTCSS works by transmitting a low frequency tone on the channel, and in order for the receiver to process this, there is a noticeable delay before the squelch opens on the receiver, and you can hear the person talking to you.
VOX works by deciding of there is sufficient speech arriving in the microphone and subsequently enabling the transmitter. There is a short delay for this to happen.
As both of these systems running together begin to incur appreciable delays, you end up finding that if you start counting, you generally get to about 3 or 4 before the other person hears you. I can imagine that in challenge events, where I guess a lot of the communication may be short "Go" "Stop" or "Winch In, Billybob" type messages, this could become quite frustrating quite quickly, and all you would end up hearing is the speaker clicking, or the "Billybob" in the last example.
Of course, in a busy competition, if there are more than 8 teams using the PMR system, there's a good chance that you'll clash with another team at one time or anther, and the received audio will also be incomprehensible.