Extension methods in a language that explicitly supports them and with good IDE support are perfectly fine. I love them in Kotlin. Extension methods as a compiler plugin that rewrites the AST... not so great. Great IDE support is a large part of what makes Java worth using, the last thing I need is to break that.
That is the generic argument against extension methods wherever they appear. They are very confusing, until you get used to remembering their existence, then you know what to search for.
Also, how about IDE support? I would argue it is at least as important as the feature itself.
This is mentioned in TFA.