The range interfaces (InputRange, OutputRange, etc) and their helper creation functions hide the @safe-ness.  This makes it problematic to use them in what would otherwise be @safe code.  For instance, in std/algorithm.d:

unittest
{
    // joiner() should work for non-forward ranges too.
    InputRange!string r = inputRangeObject(["abc", "def"]);
    assert (equal(joiner(r, "xyz"), "abcxyzdef"));
}

yields:
std/algorithm.d(3927): Error: safe function 'std.algorithm.__unittestL3923_114' cannot call system function 'std.algorithm.joiner!(InputRange!string, string).joiner'
std/algorithm.d(3927): Error: safe function 'std.algorithm.__unittestL3923_114' cannot call system function 'std.algorithm.equal!().equal!(Result, string).equal'