It seems some of the statistics and status op-mode commands haven't yet been ported with the xml/python implementation.
1.2:
vyos@crux-test:~$ show conntrack-sync Possible completions: external-cache Show connection syncing external cache entries internal-cache Show connection syncing internal cache entries statistics Show connection syncing statistics status Show conntrack-sync status
1.3-rc6 / 1.4:
vyos@equu-test:~$ show conntrack-sync Possible completions: cache Show connection tracking cache entries