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As long as their sensors correctly identify a rollover vs. another kind of accident, I think it's an acceptable risk. If you're in a rollover situation you're likely to have both hands on the wheel already. If your arm is out the window during the rollover it's not likely to fare well anyway.

Which hand signal (brake, left, right) do you most often give when having your arm out the window? Do you mean it, and follow through, or is this the old-fashioned equivalent of leaving your turn signal on?

Note: hand signals are still legal, still tested on driver exams, and (as I saw a few weeks ago) still actually used for real.

Why isn't the middle finger an option?

Your concern should be for the safety of your customers, not for whether a measure that drastically increases safety might lead to a suit in rare cases.