The Honda Goldwing has used the flat 4 for some 20 years, and I have owned 3 of them. I will never forget when they showed a silver dollar on the engine case with it running.
The low centre of gravity makes it ideal for motorcycles, and in my opinion, this makes it the perfect engine for a bike.My Goldwing 1000c.c. was actually easier to handle than my honda 750 which hade an inline 4 sideways in the frame. Most people cant feel the difference, but if you had a flat engine in your car, I think it would definetly handle better around corners.
Yes, when sitting still and revving the throttle, the bike will lean to the right side!!!!
Airplanes have used the flat 4 for 30-40 years in applications around the world, Continental, Lycoming, etc... Small compact, excellent balance, etc..
The in-line 6 has perfect primary balance.