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