The bane of the existence of most mid to high handicap golfers and very strong and high speed golfers…is getting steep and/or narrow on the downswing.
That being the case, having a very flat backswing and/or attempting a one plane swing is a recipe for disaster and leaves you very low margin of error.
Matt Kucher not withstanding, having your left arm across your chest and below your right shoulder at the top of the backswing, is just asking to be steep, narrow and…bad at golf.
Now the devil’s advocate would say lifting too much and having arm over run is bad.
Well, it is, but like in politics, the ends of the spectrum can be dangerous and offensive.
I believe while a one plane swing may be more efficient on paper, it gives you no wiggle room.
The other point to make is if you give yourself some room to shallow the arms on the way down, you may stumble into a really good swing that many would consider one plane.
However, trying to thread the one plane pinhole (I have observed) can end up on the shanks and other bad shot patterns.