Proper makiwara training will give you power to knock back a 2000 lb heavy bag, as opposed to proper bag training will give you enough power to knock back the amount you train on, ie if you train on a 100lb heavy bag, dont expect to be able to knock back a 300lb bag.
The makiwara must be properly made and trained on though, not very many people do this, most people just take a big ole 2x4, slap it into the ground and put some rope around it.
The heavy bags best aspect is the development of distancing, timing, and foot work, along with attribute training - power, speed, agility, endurance, etc...
"Everything is already, and always will be given" - Our New Pope. B