In my games (at this time, no mods), when I can finally start building megastructures, I have won or nearly won.
Building the megastructures is for flavor only. But given the pace, it's really a nerve tester.
Still not a fan of the only 1 megastructure at a time. You can field fleets of huge ships like battleships and titans, but you can't build more than
1 megastructure. If you would limit ringworlds, I could understand, but not being able to build a science nexus and a sentry array? Why not?
I can understand it's hard to balance but as I said, in my games they are never the deciding factor.