I think that item&omnia harvester should be used on Atlantis - there are maps which give special ingredients, so it’s good to pursue those with the 50% increase.
Recruits harvester should be preserved for S1 8-7 IMHO.
Iron Harvester seems best to use on S1 17-1.
Food Harvester not so sure, I think there was a S2 15-9 on Hard discussion, but I’d let more experienced players have a saying on this one.
Also, in my book the right time to use a Harvester is when:
- There is no major event running with World Energy flags such as Atlantis Rising or Challenge event
- There are no good quests present or have been just completed
- I have all my World Energy restored to the fullest
Meaning the recent Valhalla portal opening for example was such a good time to use a Harvester or two, since it was only a portal and no event attached to it. Other events which are nice to work together with the Harvesters are the Ninja Tower and the Mythic titan since both use different than WE flags.