If the engineer who wrote the code understood the problem well and was a decent engineer, then the code will be maintainable no matter what the language is. Understanding the problem well is very rarely achieved.
The issue is not "is the code maintainable" or "does Go lead to unmaintainable code".
The issue is, do I wish to spend my days debugging code and writing more code, in Go? I used languages with Go's limitations back in the 1990's. I don't want to go back.
Writing and debugging code in Go is extremely more pleasurable than debugging, say, python. I write Go all day while our data scientists write python. Unfortunately python gives those guys enough rope to hang themselves and me too. It's simply harder (not impossible of course) to write unmaintainable code in a limited, static language like go.
Python in a nutshell: What you gain in flexibility and terseness, you more than lose in needing 100% unit test coverage, because every line could fail to execute (with an error that many other languages would find at compile time).
The issue is, do I wish to spend my days debugging code and writing more code, in Go? I used languages with Go's limitations back in the 1990's. I don't want to go back.