From README.md, things treated as unsafe:

If a type implements the json.Marshaler or encoding.TextMarshaler interface (e.g. json.Number).

For encoding.TextMarshaler it seems to hold:

package main

import (
	"encoding/json"
	"fmt"
)

type Foo int

func (s Foo) MarshalText() (text []byte, err error) {
	return []byte("\"foo\""), nil
}

type Bar struct {
	X Foo
}

func main() {
	x := Bar{X: Foo(0)}
	out, _ := json.Marshal(x)
	fmt.Println(string(out))
}

...gives

foo.go:20:12: Error return value of `encoding/json.Marshal` is not checked: unsafe type `main.Foo` found

However, for json.Marshaler it does not seem to hold; if MarshalText is changed to MarshalJSON in the example, thus making it implement json.Marshaler, no error is provoked. This is with 0.2.2.