There's never really been an issue with MC itself in a wet / outdoor location. You're doing just fine until you get to the end of the cable.
Why? Because every MC connector I've ever seen has been marked as for 'dry locations only.' There's the problem.
Were it not for the price of the (weather-tight) fittings, I suspect we'd see a lot of UF used as HVAC whips.
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As I learned the trade, the only material considered for such whips was metal-lined "Sealtite." I will admit that there is still a role that only such a method can meet.
Yet, I wonder if we're trying too hard to be 'perfect?' For the applications we're discussing, we're using "normal" 20- or 30-amp 120 power. Even the use of 240 in residential air conditioning is becoming rare. It seems a natural application for a less elaborate method - even (gasp!) a cord and plug.
Frankly, I'd have greater faith in a cord & plug than I currently have in the typical installation, where the flex is falling out of its' connectors and the connectors are hung on a disconnect that seems to be made of foil, and is usually improperly mounted.(How often do you see one of the mounting holes wide open to the weather?)
