Bill,
The vocopro has pull out channels that can be replaced in the field. You can pull a single radio, send it in for repair, and keep right on going with the other 3 (or 7 in the case of the 8800).
While I wouldn't dream of using it for critical events - for karaoke I find it 'acceptable'.
As for multiple reciever units - Audio Technica makes the 5000 series as 1U with 2 microphone receivers... I wouldn't think twice about using one of those.