The ones telling you to mount the ceiling fan bracket directly into the joist are correct. This solution will be stronger and safer than any "fan-rated" box you can buy. Use 2.5 or 3" construction screws and make sure they are solidly into the joist

Most fan mounting brackets have slots on either side so there is plenty of space to put the screws just outside the pancake box.

Avoid using the little metal tabs inside the pancake box. They are not strong enough and over time there can be metal fatigue because fans do wobble and vibrate.