I really think fusion reactors aren't that great. I don't understand why the AI would build them first thing....I saw them building them when I was taking over a race, and I think they were trying to sabotage my economy....
if I am correct, fusion reactors give you 10% more economy to the planet, and cost 3 maintance, so if that fusion reactor isn't producing at least at the same level of your factory, then its not worth building them......your better off for higher production to produce a factory....
if I had 9 tiles on a planet, seems if I built all factories and one fusion, I might break even if I'm lucky.....and have the fusion acting as a factory. If I want to build anything else besides factories, it makes it harder for a fusion to be very effective...
granted the higher level fusion types are more effective and probably better and easier to break even with them....
This also seems to be the case with the research one also, why bother building a 10% bonus instead of another research facility type building.... unless you have at least 9 other research buildings already on planet.....