How do these distributed build tools work across the Internet, i.e. outside of a local LAN office setting? Does the increased latency and slower bandwidth become a bottleneck?
Bandwidth and latency effects will have some effect on how well it will scale, but on a reasonably high latency and low RTT connection it's probably still faster than purely local compilation. Sending work over the network takes time but not much CPU, so the local CPU is freed up to do other work.
So all in all, I wouldn't say works outside of a well-controlled cluster on a LAN.