CLOS itself is great, however the decoupling of data and methods is something at times I find leads to harder to understand code.
I think that might be familiarity rather than anything fundamental. I often find the coupling of data and methods to lead to harder to understand code; if I want to define a new method on an object in Java, I have to either subclass it or modify the class itself. Even if the method is ancillary to the core functionality of the class. That seems strange to me.
> I want to define a new method on an object in Java, I have to either subclass it or modify the class itself.
objective-c/swift/kotlin/c# all support them to varying degrees (but sadly not java afaik)
A quick read of the wikipedia article makes it look like in C# you can do this, but not in Ruby.