I think one problem with statements-as-expression is that is not always evident which should be their result.

for instance:

    bool r = if (test()) {
      false;
    } else {
      true;
    }

what r should be set to? true or false?

I don't see the point of adding such complexity.. it's waaay more clear to be explicit

    bool r = test();
    if (r) {
      false;
    } else {
      true;
    }

or, if you intended the other way

    bool r;
    if (test()) {
      r = false;
    } else {
      r = true;
    }

I guess that's why you need to be strictly correct, or highly opinionated, to design a language.