I agree that mutability constraints are a major weakness of Go. Interestingly, they did make string immutable and added []byte as a mutable alternative. It's unfortunate that string and []byte are so similar and yet it's impossible to treat a []byte as a string without copying (with the exception of looping over runes). This leads to massive code duplication and/or lack of functionality (byteconv where are you??). Just look at the strings, strconv and bytes packages. This whole area of the language is messy.
The funny thing about Go is that it makes up for its long list of weaknesses with essentially one single strength. You can actually read other people's code without much introduction to the concepts used in that codebase, because the number of possible meanings of any particular expression is much smaller than in other languages.
There is so much talk about Go being for dumb, second rate, corporate developers, because that's what Pike essentially said at one point (perhaps without thinking first).
But in fact, it's not the developers who are dumb. It's the process by which large corporations employ and dispose of developers. They are thrown into some project and expected to "hit the ground running". There's no time for explanation. So what they do is read code to acquaint themselves with the codebase and hopefully become productive before they move on to the next job. And that is the one task where Go really shines. Reading arbitrary pieces of code.
Of course powerful abstraction features eventually make reading code easier as well, but only after having learned the abstractions created for that particular problem and codebase and only if those abstractions are very carefully crafted.
Powerful language features help writers of code long before they help readers. And that, I believe, is essentially the dirty secret that Go exploits.
We all want to be brilliant writers of code when in fact we are often readers poking helplessly at half understood code to make something happen. We even forget our own abstractions once we haven't looked at them for a couple of months or even weeks.
The problem with Go is that it not only acknowledges this state of affairs, it also enshrines it.