Flat 4's ARE balanced, or at least as far as people care about. They "wobble" which means they love to pitch up and down. Ever wonder why subaru seems quite anal about pitching stoppers?? Anyhoo, they don;t have those nasty vibrations, a inline 4 or unbalanced V8 ( external or internal) has.
Thats also why I want to build a 10K+ rpm ER-27 when I have a little cash. Its a flat 6, with a 2.62 inch stroke, and a 4.6xx inch connecting rod. That equates to dog slow piston speeds and forces at low rmps, and normal ones up high. Then factor in the natrual balance so I can make the whole rotating assembly very light and balanced.