There are really good reasons to choose Unity over Godot, but C# isn't one of them. You have to learn a whole new API, framework, workflow, etc. Learning GDScript is the least of the issues.
This isn't always true. C# outperform GDScript. Even GDScript 2. GDScript is great for quickly programming certains events. But if you want to create complicated systems that using require thousands or millions of data points, you will notice the speed difference.
And C++ outperforms C#, so native modules are always an option where you really need performance. Yes, you have to build for every platform but most projects really won't need it - but its there so you know you have options when you do. GDNative is also an option to avoid building the core engine, and its still at least as fast as C#.
Except most Unity users don't want to write C++, if they wanted to, they would have chosen another engine in first place.