I thought this feature was a bit odd once I first started using Nim. But by now I'm a huge fan, and I'll explain why. But first I'll just say that yes, it is slightly harder to search for identifiers. But libraries will stick to either snake_case or camelCase for their identifiers, so as long as you know which one it is it's not that big of a deal. The benefit of this style insensitivity though is really nice. I programmed in Python and C for many years before trying out Nim. And in both languages I've run into libraries that does the opposite of what the style guide recommends. Some people just prefer one style over another and will do their libraries in their preferred style. The problem is that this meant that my code ended up as a hodge-podge of the different styles. This was definitely more prevalent with Python code, but I've run into it with C as well, especially with micro-controller programming. With style-insensitivity however this is a complete none issue. You can write your code in whatever style you prefer, and not care about what dubious stylistic choices the library maintainer sticks to. To me this far outweighs the small inconvenience that it brings (especially if you take into account editor tools).

I feel most people who read about style insensitivity imagines all Nim code being written as a crazy mix of styles, while in reality Nim code is often more consistent in style.

And the tabs vs. spaces issue is simply to alleviate the issues with "how many spaces are a tab" which is impossible to guess when compiling code. The replace filter you mention above essentially just explicitly specifies how many spaces you intend a tab to be. That being said I would've liked it better if it forced tabs, that way you'd have one tab for one indentation level, and everyone could choose their own preference of indentation.