Depends a lot on the boom pole. One company I know has a boom pole formerly rated at 1500 lbs built from light gauge square tubing. Several people had this thing fold up part way out of the engine compartment trashing their front end. They have since de-rated their unit to 1000lbs.
Keep in mind, ratings for boom poles are generally for static loads. When you start moving with the load, it becomes a dynamic load. That little bump you travel over can make your 1200lb engine assembly act like 2000 lb.
Also, many times, failures with boom poles are quite sudden. In other words, instead of bending a little, they fold up completely. Once tubing (square or round) bends enough to start a kink, the structure loses most of its integrity. Try it with a paper towel tube and you'll see what I mean.
Pull an engine with a boom pole? Depending on the boom pole - sure. For safety sake though don't get under the load with your body or hands and arms.