DI for testability adds complexity to code and reduces readability. In Ruby it's almost always unncessessary to use DI because in Ruby you can stub at runtime.

In other statically typed langues like Rust the type system itself eliminates the need for a lot of these tests but at the cost of mental overhead of expressing your logic in a way which will satisy the type system.

What jashmatthews said mostly. It adds bloat and isn't very readable. It's another "ceremony" that together with types, interfaces, generics etc increases lines of code. I can see the benefits for huge projects, but I don't like this way by default out of the box.