But that just brings me back to my original point - in my games there is a constant lack of mineral-rich planets, and lots of planets with 1-2 mining Districts. So, again, the Ecu becomes ueseless, because what little ore I have must go to Goods to satisfy the population, and nothing else - making the planet a net loss.
I don't know about you, but I like to take the mineral bonus trait for my species, and that helps me always have an abundance of minerals. Plus, you should also be getting minerals from mining stations, but also the Ecu is still not a net loss even with the scenario that you mention. The Ecu is better at producing consumer goods and alloys than any other planet. If you get an Ecu, you can literally offshore all the consumer goods production you were using on your mineral producing worlds. In case you didn't know, there's thing called planet specialization. Now specialization can basically just give you extra stuff out of thin air.
Let's list them and their bonuses so that you understand, I'll only talk about the ones for normal empires.
Empire Capital +5 Stability, +10 Amenities, +100% Government Ethics Attraction
Rural Planet +2.5% Worker Output
Urban Planet +2.5% Specialist Output
Mining Planet +5% miner output
Farming Planet +5% farmer output
Generator Planet +5% technician output
Foundry Planet +5% metallurgist Output
Factory Planet +5% artisan Output
Refinery Planet +5% refiner/translucer/chemist Output
Research Planet +5% researcher Output
Fortress Planet -10% Orbital Damage, +10% Defense Army Damage
And then there's Resort worlds, Penal worlds, Thrall Worlds, etc.
These are bonuses for specializing your planets but then you also can research tech that you allow you to produce buildings that specialize even more.
For Food and Minerals you can build buildings that give +15%, and then upgrade it to +25% with more tech
You can build a building that gives +15% Artisan output and +15% Metallurgist Output
There's a building that gives you +15% Researcher Output
There's a building which gives +20% Trade output, another that gives +5% Slave output.
But here's my most important point
All Ecumenopolises have +20% Output for ALL jobs and +50% growth speed.
And you can build ANY or ALL of the buildings you can build on a normal planet on an ecumenopolis.
If you specialize your planets like crazy, you get stack some multipliers and get a whole bunch of resources from thin air.
If you really wanted to, you could produce a small amount of food or minerals on an Ecu, because of the buildings which gives +15/+25% to food and minerals also give +1/+2 jobs, respectively.
But my whole point is that you can produce 20% more everything on an ecumenopolis for ZERO extra cost. On a normal planet, you could specialize it to produce +20% Consumer goods and 15% alloys, or +20% alloys and +15% consumer goods. On an ecumenopolis, you can produce +40% consumer goods and +35% alloys or +35% consumer goods and +40% alloys.
Plus the districts on an ecumenopolis are special, there's one for +15 housing/trade, one for consumer goods/+10 housing, one for alloys/+10 housing, and for one unity/amenities/+10 housing. and you can STILL BUILD normal factories. If you're still not convinced, then you've stuck your head in the sand.