The mid-game is now more interesting as planets are far more detailed, but if you ever get into 40+ planets the game turns into a micro nightmare since planets are never truly "done" like they were before.
While I wouldn't say it's a nightmare, Mastikator is basically right.
In previous versions of the game, planets eventually became "done." You might shift their output later, but once they were full and have all buildings upgraded, you could just ignore them.
Megacorp is not like that all. Most planets go through an initial period of being rural resource generators, then they go through an urbanization process of some kind. The rural planets, even when "full", are focusing on basic resources. But then we go through urbanization and start focusing on buildings for alloys, consumer goods, and research. But then I start converting resource districts over to city districts.
Then some planets go from "urban industrial hell holes" to "urban banking center hell holes" when they convert buildings over to commerce buildings, and I suddenly want to support 300 POPs on one planet on clerk jobs.
Then I start building wonders, and 50 planets are shipping POPs to to the Ringworld every year, and the ringworld is desperately trying to employ them all as I constantly build districts there. Oh, and did I mention ecumenoplis? That's a whole other rats nest of development to get into. Great, so I'm building an ecumenoplis. It's finished. Now I start offloading alloy production to it, so I don't have to spam alloy buildings that use special resources. But since it will have 300-400 POPs on it, I have to make careful decisions on how to set it up. And since it has a special POP growth bonus, I really don't want to turn it off, so I end up resettling POPs from it twice a year once it's "full."
Then I get a new technology, and it's time to rethink half the empire's buildings.
Gestalt empires have no problem allocating POPs to jobs. If you don't want someone on a job, just turn it off. Easy peasy. But for regular empires, there are issues where you end up with specialist POPs that won't demote instantly to worker jobs. So, you can't just carpet planets in buildings and districts, or you end up with a bunch of specialists and no one working the mines and farms. Then you enter the spiral of economic death.
It's worth pointing out that because housing and amenities are not global resources, you can't just "template" planets and say "Oh, this is a 15 tile planet with X number of mining districts. I will build it the same way as the other 15 tile planets." Thanks to the effect habitability has on amenities, and how various jobs generate amenities, and how many amenities in various jobs use amenities, it's more complicated.
These aren't bad things. In particular, pacifist empires have more to do. But it also means that the last time I fought a crisis, I just closed the part of the outliner that displayed planets. It was too distracting to see all those employment and housing indicators while I tried to defeat the Unbidden. I'd only look every two years or so and correct as best as I could while I waited for building changes to take effect.