I've never used mustard so I'm not in a position to have a say on it, but from what I've read about meat tenderizers, and acidic sauces, they can only penetrate so far into the meat being cooked. Unless you brine or marinate the meat for a period of time you really aren't doing much tenderizing via that process. I've only read that if you're in a pinch, using mustard will help the rub penetrate a little better. But if you have the time, doing a dry rub and then wrapping the meat in plastic and allowing it to sit 3+ hours in a fridge, or sit out at room temp for 1 hour, that is the better way to season your meat.
Like I said, I'm not expert. When I made ribs in 2 restaurants, we only used dry rubs.
Oh and the only bark I really like on ribs is the one BBQ sauce makes:
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