Brook takes shots well, but as I've said, GGG is near flawless. Sure, he can be flat footed at times. I actually think he's capable of leaving himself exposed on occasion, too, by over-reaching for punches. But we've only seen Murray take him far into a fight.
And Murray is a durable, tough and big old boxer. Brook is big for a 147lb fighter, as I've said, and likely hits the ring in the late-150s anyway, but it's a big task and a big ask.*