Companion is a singleton. Static methods are visible through subclasses, companion objects’ methods are not. Static methods are procedural, companions are object-oriented.

I'm not sure there is any use of static methods being visible through subclasses except for confusing readers of the code using them like that.

There may be some utility static methods, but one may think they are overridable, which is confusing.

Ok. There is one use: having your own subclass of assertj's

Assertions

and *-importing it to get all functions.

First way (subclassing) is similar to declaring constants in interface and implementing it (so constants becoming visible through class which breaks encapsulation). Importing is better, and Kotlin allows it too.