In practice, both (qemu and hw) buildbots are useful. qemu is a bit faster and easier to scale, hw is the ground truth.

Google Native Client buildbots are exactly like that:

qemu: http://build.chromium.org/p/client.nacl/builders/precise_64-...

pandaboard: http://build.chromium.org/p/client.nacl/builders/oneiric_32-...

And yes, Panda is very unstable under heavy load. :(