Just about every new car has the Latch system in the back seat, makes it easier to install the seat safely than using just the seat-belts alone. Subaru suggests the outboard seating positions because the cushion in the middle sometimes makes it difficult to belt the seat in snugly although other Subaru owners have found ways to get the seat to fit correctly in the middle.
The middle position has been long favored but some seat designs aren’t flat in the middle and make it harder to install the seat where it won’t move around.