[bob1029]

Unity can definitely support scenarios involving 100k+ scene elements. The problem is that you cannot place the art team any meaningful distance away from the development team when using something like ECS. Game objects + components allow for a high degree of decoupling between the art & technology teams, assuming the total # of scene elements is well bounded (<10k).

A custom engine is probably not a bad idea for a city builder. I feel like ECS is a good middle ground because you would need some pattern like this anyways and it would serve as a good reference if you decided to go fully custom.

I would challenge the notion that we couldn't develop an effective city builder within the constraints of a reasonable # of game objects. Players may be compelled to accept an experience that has a significantly smaller world size if the richness within each unit of world space is much higher.