I used this piece of software for many, many years - it's pretty nifty - but it's a toy app. I personally wouldn't recommend sticking with it personally and certainly not in FreeNAS core. The third party versions for BSD/Linux I found to have very little activity on the repos and had serious bugs like memory leaks which would lock up our servers.
Bjango also update the app very sporadically and to be honest there are better solutions like SNMP, Zabbix, Nagios which are more mainstream anyways and which I'd recommend over iStat Server. Zabbix in particular is incredible (if a little tricky to setup).