buildfarm and "rolling release" distros

I've always been a bit reluctant to accept buildfarm members that are 
constantly being updated, because it seemed to me that it created 
something with too many variables. However, we occasionally get requests 
from people who want to run on such platforms, and I'm also a bit 
reluctant to turn away willing volunteers. We have one such application 
now in hand.

What do people think about this. Is it valuable to have? Do we have 
enough stability from the buildfarm members that are not auto-updated 
that we can accept a certain number of auto-updating members, where, if 
something breaks, and it doesn't break elsewhere, then we suspect that 
something that got upgraded broke the build?

I'm also not sure how to designate these machines. The buildfarm server 
metadata isn't designed for auto-updating build platforms. But no doubt 
if necessary we can come up with something.

cheers

andrew