Wouldn't a saber with both Excalibur and Avalon be Nigh Unstoppable?
Canonically quite stoppable; she can in fact be defeated by sneaking into her home, stealing the scabbard, and tossing it into a lake where she'll never find it, then defeating the now-vulnerable Saber in ordinary combat, such as running her through with a spear.
Since, y'know, that's why she's the once and future king resting on the isle of Avalon healing from her wounds until Britain has its hour of greatest need, rather than the once and present king reigning from Buckingham Palace.
