Very unlikely for a spectator to be injured by a cricket ball.
A key reason for this is that the cricket pitch (i.e., the area where all the action happens) is in the center of a large oval, and usually at least 60 meters (about 200 feet) from the fences, where the closest spectator would be sitting.
The greatest danger to spectators in baseball is from hard foul balls that go wide enough to miss the protective netting behind the catcher, and from hard-hit line drives that go foul down the left- or right-field line. I saw a spectator caught square in the face by a line-drive at Camden Yards about 6 years ago, and she had to be carted off on a stretcher. These types of shots come incredibly quickly, and often catch spectators unawares. Anyone who sits in certain areas of the stands, down the first base and third base lines in particular, knows that they need to keep an eye on every pitch. Even for those who are paying attention, a screaming liner into the stands is a dangerous thing.
There is simply no equivalent to this in cricket. The cricket equivalent to a foul ball is an “edge,” but the different shape of a cricket bat and the way that cricket strokes are played (more often with the bat held vertically, rather than horizontally) means that an edge almost never travels far enough to get anywhere near the spectators. A few top edges have gone for six runs (i.e., over the fence) in the history of cricket, but it’s a very unusual occurrence.
When a ball does clear the fence in cricket, it’s usually because the batter has hit a four, and the ball has bounced one or more times on the field before bouncing over the fence and into the crowd. Sort of like a ground-rule double in baseball. The other possibility is that the batter has hit a six, a shot that clears the boundary on the full, like a home run. In both those cases, it is very unlikely for a spectator to be injured by the ball, because the spectators usually have plenty of warning that the ball is coming.
And it’s similar in baseball. When was the last time a baseball fan was injured after being caught unawares by a ground-rule double or a home run? Any injuries resulting from these shots are much more likely to be hand injuries from trying to catch the ball without a glove, or more general bruising caused by multiple people fighting to get the ball.
Added to all this is that fact that far fewer balls are hit in the air in cricket. The nature of the game, the shape of the bat, and the emphasis of batting training all work towards keeping the ball on the ground as much as possible. A perfect cover drive or pull shot or square cut is one that is belted downward toward the ground so that it rolls to the fence without the opposition having any chance to catch it on the full and get the batter out. Of course, balls are hit in the air quite frequently, sometimes by accident and sometimes on purpose, but “ground balls” are the norm rather than the exception. It possible to watch a whole day of cricket—especially test cricket—and never see the ball enter the spectator areas.