Yes, horizontal line arrays are an issue if you maintain a coherent wavefrom. The radiated sound would be the same as a vertical line array except rotated to the horizontal plane. The sound would be limited primarily to the length of the array and such.

Now the Yamaha array operates differently in that each driver haws a specific delay or phase so that it can recreate a home theater experience in a normal listening room. The multiple channels for HT are determined by reflecting the sound from various drivers off the walls of the room.

I haven't heard the Yammie array but I suspect by its design that it would be dependent on room reflectivity and layout. What would sound great in a moderate sized room would perhaps present an issue in either smaller or larger rooms. Still you can see that this design has merit within it potential limitations as to room dependencies.

Jim