the problem with grass bunkers is manage playability : too thick means unplayable + time lost searching you ball, while not thick enough means too easy, definitely easier than sand bunker (at least for most of us), but also easier than shortly mown fairway because your not afraid of shanking or hitting it thin. So with grass bunkers, the difficulty of the course will really change depending on the season and thickness of the rough.
If the ground on the golf course can be kept bumpy and rolling, I think the most difficult green surroundings is still short fairway. It requires a lot of guts if you chipp and a lot of touch if you putt.
If you're on a more wet land, I think a mix of sand and grass bunkers brings good variety.