A good question, as I like it and don't like it. It is without a doubt better than dynamic types.

On the pro side it removes redundant writing the type again and again. On the con side it is almost impossible to see what type with what functions the variable in front of you has without an IDE (which you don't have without cloning the repo) or without an already deep knowledge of the code.

Same with extension functions (free functions which can be added to a type almost anywhere in the codebase). Very useful (Kotlin is a great example for this) but also confusing when you want to hunt down where this piece of code is coming from.