There are definitely some advantages in developing your game logic in an agnostic (non-Unity) environment.
Pros
It forces you to think about game design more purely. I find it is much easier to think about game design, whether on a board or on a computer screen, if I don't also have to think about writing or using an engine.
It is easier to write and run tests, though I hear there are testing frameworks for Unity.
It is easier to simulate your game at faster than real-time. Balance and mechanics in "economic strategy" games are comparatively more chaotic than in other games: the effects of adjusting Ability X are often less localizable and less generalizable than in first-person shooters for instance. Therefore balancing this type of game is a much more time-consuming process. When writing AI or tuning numbers, being able to simulate many games (outcomes) generatively, automatically, at faster-than-real-time, and to finally output statistics, can be incredible useful.
MonoDevelop for Unity is only ok. If you write your game logic in a pure .NET library, you can use Visual Studio which has a more mature debugging environment.
And as you alluded to, to validate game moves on a server you probably want all or some of your game logic to be running in an environment not dependent on Unity.
Cons
If your game model depends on physics, or any other capability already offered by Unity, you must recreate it or use some other library that you can link against or build.
In general you might find yourself rewriting certain basic constructs in your own library, and writing wrappers or converters to go to-and-from yours and Unity's.