Will a pineapple marinade reduce a beef roast to paste?
Pineapple contains Bromelain, which is "one of the most popular proteases to use for meat tenderizing."  Since it's sold as a meat tenderizer, I'd say it really just depends on how long you marinate with it -- it's possible to over-tenderize something.

This warns about over marinating, and mentions recommended times:


  The same process that tenderizes steak can also break it down into mush if you marinate it too long in the pineapple, and it'll start distorting the meat's color and taste. For cuts thinner than 1 inch, stick to about 10 to 15 minutes of soak time; for moderately thick steaks of 1 to 1 1/2 inches, marinate for roughly 15 to 20 minutes; and for thicker steaks, marinate for about 30 minutes.