It is, as long as it's set as the default app - that's the whole principle it relies on: if the default app is missing or unusable, it's going to be a problem regardless of whether that's Wii U Menu, Brain Training, some alternative to Wii U Menu for developers, or something completely unrelated
By percentage of reported bricks with a known cause - that's rather unlikely (though of course possible) - the vast majority is caused by cockpit error
Of course whether that's a worthwhile tradeoff is inherently subjective - aren't we modding our consoles to escape someone telling their way is the only right way?