I would argue that UE5 is even less suited to a game like this than Unity is. Unreal certainly has impressive rendering tech, and it has designs towards increasingly becoming a generalist engine, but it is clearly designed with certain genres in mind (i.e. 1st and 3rd person games like RPGs, Shooters, Action games, etc.). A city-builder in UE5 would present a whole host of other challenges, and many of the high-tech rendering features would likely be overkill. Not to mention, Unreal games have notorious performance issues of their own--though there is dedicated effort to resolving those.
Unity is designed more as a general engine, but it comes with a lot of baggage in terms of half-baked features and optimization difficulties. As the author mentions they really unlocked their potential with implementation of Unity's ECS framework, but they were still chained to Unity's rendering tech, which has been underdeveloped for several years now.
My observation tends to be that simulation games are the ideal case for custom engines. While there are some commonalities across games, compared to many other game genres, they don't get a lot of benefits from standardizations. Sim games often end up kneecapped by trying to conform to existing engine frameworks instead of spinning up something optimized to the way their systems work. It requires a lot more technical know-how than an action-adventure game or a platformer, and the up-front cost to developing your own tech is an order of magnitude compared to using out-of-the-box solutions. I think with the massive success of C:S, Colossal Order was in an excellent position to try something ambitious.
Maybe with open-source tools like Godot having more flexibility in their frameworks, where you can just get the parts you want (rendering approach, etc.), it'll be easier in future to develop more specialized custom tech for games.
City builders, and certain classes of RTS are really the last major forms of games that really are very poor fits for modern off the shelf engines.
Honestly, trying to build one in either Unreal or Unity is going to be a painful experience with challenges likely surpassing having just built the engine you need in the first place.
But Engine selection is really only a technical decision less then half the time these days anyways.
Frost giant is building stormgate, the spiritual successor of starcraft 2, with even better/smoother gameplay than sc2 with Unreal Engine 5, they picked what parts they reused such as the renderer and built the underlying stuff from scratch, but still
That seems like a slightly different scale of simulation, though... a SC-like RTS with a few hundred units at most, vs tens of thousands of citizens in Skylines?
I'd more compare it with something like Dyson Sphere Project, which can scale from a few factories & conveyor belts to many planets and thousands of things (resources, drones, belts, arms, whatever) on-screen at once. It's also made in Unity and has great performance and terrific graphics.
Unity is designed more as a general engine, but it comes with a lot of baggage in terms of half-baked features and optimization difficulties. As the author mentions they really unlocked their potential with implementation of Unity's ECS framework, but they were still chained to Unity's rendering tech, which has been underdeveloped for several years now.
My observation tends to be that simulation games are the ideal case for custom engines. While there are some commonalities across games, compared to many other game genres, they don't get a lot of benefits from standardizations. Sim games often end up kneecapped by trying to conform to existing engine frameworks instead of spinning up something optimized to the way their systems work. It requires a lot more technical know-how than an action-adventure game or a platformer, and the up-front cost to developing your own tech is an order of magnitude compared to using out-of-the-box solutions. I think with the massive success of C:S, Colossal Order was in an excellent position to try something ambitious.
Maybe with open-source tools like Godot having more flexibility in their frameworks, where you can just get the parts you want (rendering approach, etc.), it'll be easier in future to develop more specialized custom tech for games.