The rear window does get dirty fairly quickly but not to the point of preventing vision, however I don't drive on muddy roads so I can't comment on that aspect.
With the roof fully down the interior mirror will just show the top of a following car so it's not much use. That only applies if the roof's fully down obviously, if you only open the roof to the 'spoiler' position (above the rear window) there's no issue.
The small back window also inhibits the view when reversing so it's best to learn to reverse using the exterior mirrors alone with and without the roof down. It takes a bit of practice with the convex mirrors to get a straight result
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Definitely consider getting reversing sensors fitted.