When looking at the profile of a running query, it's impossible to tell the degree of accuracy. We've seen issues both with instances not checking in with the coordinator for a long time, and with hung instances that never update their counters. There are some specific issues as well, see IMPALA-5200. This means that profiles taken off of running queries can't be used perf troubleshooting with confidence.
Ideally, Impala should guarantee counters to be written at a certain interval, and warn for counters or instances that are out of sync for some reason.