Harvesters kick ass so badly it's not even funny. IMO, they're the most favorably unbalanced race for the player, but they're good for taking out your aggressions.
There are some green worlds out there for them, and they do well enough on most worlds that they're impossible to slow down. They do well enough in manufacturing and environmental (a really advantageous combo, since much less money is lost to pollution cleanup) that they can maintain a good enough fleet to prevent blockading of existing systems and to relentlessly expand.
The trick with Harvesters is to play hyperaggressively - you can't do balanced planets, because there's not enough that are useful enough to you. You have to have specialty planets, and keep things going by having enough forces to prevent or quickly eliminate blockades. The fact that MoO3 allows instant transport of bioharvest and minerals, and that you don't have to build or directly secure transport fleets, just adds to the Harvester's ability to dominate.
I don't hesitate to take red 1 or red 2 worlds that are mineral rich or very rich, and use these primarily for mining. Any green worlds (until I have enough), I bias towards bioharvest), and yellow worlds get the emphasis on industry and research.
I only do colony ships where there's green planets - having to do three for a red planet (or four for other races) is silly when you can get 5-8 outpost ships for the same total production, then just migrate population. You'll have more planets, faster, by skipping colony ships for yellow and red planets.
Another nice trick with the Harvester manufacturing / environmental advantage is making "system outpost" ships. These are dirt cheap, and you can populate out a system quickly if you find one decent planet, get one working industry DEA with no mineral or pop shortage, and then crank system outpost ships to put outposts on all the remaining planets in that system. Then set migration orders on the more desireable planets, and you get lots of planets fast and cheap to get minerals and production out of.
It doesn't take long into the game before the Harvesters are dominant and unstoppable. Screw diplomacy - the only thing I use the diplomacy screen for is intel, by using the exchange items dialog to see who has what that I might want to take from them. Then conquer or send spies, and that's that. Slurrrrrrrrrrrrrrrppppppppp and burp.