Problem when using VoiceProcessingIO audio unit on Mac

I'm writing voice chat Mac application and I am trying to use VoiceProcessingIO audiounit.
I noticed that:

1. I still get echos

2. sometimes it takes time to allocate the audiounit (more than 2 seconds)

I also noticed that I must use same sample rate for input and output. Is there a reason for this limitation?

I wondered, does VoiceProcessingIO is really working well on Mac?
Any suggestion what to look for?