the freebsd em driver periodically hangs when running in a VM on all versions of esxi after (but not including) build 799733 (ESXi 5.0 GA). The symptoms of the problem are that the freebsd vm drops traffic on the affected interface for a period of time. The kernel reports errors:
Apr 22 19:12:08 xxxx kernel: em0: Watchdog timeout -- resetting
Apr 25 07:38:18 xxxx kernel: em0: Watchdog timeout -- resetting
Apr 27 23:29:35 xxxx kernel: em1: Watchdog timeout -- resetting
Apr 29 18:03:29 xxxx kernel: em1: Watchdog timeout -- resetting
May 5 05:28:38 xxxx kernel: em1: Watchdog timeout -- resetting
FreeBSD 8/9 were particularly badly affected by this, to the point that the em driver was totally unusable on those versions when running as vm after esxi build 799733, i.e ~30% packet loss for pings. The intel em driver update in 10.0 helped quite a bit, and anecdotally the problem seems to be less bad on ESXi 6.0. Now the figure seems to be down at around 1-2% pktloss over extended periods of time.
However it's still there on 10.1 with all recent versions of esxi, and prevents freebsd from being able to function properly as a router when using the em interface.
The problem also affects freebsd derivatives (e.g. some versions of JunOS Firefly / vRR / vMX, etc).