The issue highlighted, at least for UTF-8, is that Go just _assumes_ everything is UTF-8. It makes seemingly no effort to actually _enforce_ UTF-8, leading to silent mistakes like with the file path.
Yes, that _is_ surprising! The points made by the OP are exactly the opposite. I believe the language itself is redeemable, but it's the implementations of many features that are just so faulty.
To take your example, yes it's great that it makes UTF-8 as the primary encoding for strings, but then the implementation in the core and in the ecosystem completely fails in enforcing or honoring that.
Edit: please see the filename example in the post to see the failure around adhering to UTF-8.