The folks at "America's Test Kitchen" found that it requires a higher temperature to overcome evaporative cooling that happens as the juices evaporate at lower temperatures. I have also been successful with 350 for 90 minutes (to get the meat to the temp where drying is not a concern) and then lowering to 275 or 250.
As noted, my son (who is a 12 hour, low-temp grilling fanatic) was surprised at the tender, juicy outcome. Another benefit is that this requires little input on the part of the chef!