Ski, it seems you're an engineer.
Yes, I agree that 4-cylinders are also not balanced, but balance shafts go a long way to solving the secondary imbalance. But IMO it's not a panacea: There's a thread in the B5.5 Passat forum about a string of problems with wearing of the hex drive of the oil pump. What does this have to do with balance? Well, in the engine in question it's driven directly off the balance shafts, which are in turn chain-driven off the crank. While I don't know if anyone has authoritatively found the root cause, my belief is that the torsional harmonics of the balance shafts is causing that wear. The balance shafts of the B5.5 Passat 2.0 TDI is a sight to behold because of their
hugeness! Blame heavy reciprocating components, a long stroke and low rod L/R ratio for that.
OTOH, the ones installed in the newest BMW 4-banger Diesels are things of beauty in comparison. They are far lighter, helical gear-driven off the flywheel-end, vertically-offset and turn in roller bearings to minimize friction. These little details, and the aluminum crankcase, speak to me as an engineer as to the superiority of BMW engine design to VW's (that's saying a lot because I'm a lifetime VAG fan).
Back to inline-5s: the imbalance mode depends on the firing order and therefore the placement of the crankpins. One layout (1-5-2-3-4) leaves a large primary rocking moment with a small amount of secondary imbalance, which can then be smoothed out with heavy crank counterweights and a 1X balance shaft rotating the other way and putting up with the remaining secondary buzz, while the more common approach (1-2-4-5-3), as done by VAG, Volvo, MB, etc., is to have a small primary rocking moment and a large secondary, but saying to hell with them both and leaving them alone without any balance shafts.
I also believe V8s have a lot of advantages, as do inline-6s and boxer-6s for different reasons and different applications. I think a moment of epiphany happened after seeing the VW balance shafts above, and I'm actively involved in researching better ways to balance engines... One of my biggest banes is the secondary imbalance -- primary is fairly easy to solve -- and I'm working on a potentially patentable design to eliminate the former. If this is the case my dream is Ferrari-sounding flat-crank V8s of any bank angle becoming a lot more commonplace.