Most V-8s are "cross plane" to begin with.
Only a few of them exotic cars get flat plane cranks.
A flat plane crank will allow the engine to accelerate slightly faster - so if you have a 700hp Cosworth in your F1 Lotus, and are making 40+ shifts per lap, it makes a fraction of a second difference. And that matters in F1. If I'm not mistaken, it actually costs power (with fancy tuning, you can get the power back, but it has been theorized that the Ford Mustang flat plane engine would have made more power as a cross plane. I just wouldn't sound as exotic....
But for the rest of us mere mortals, the reduced harmonics of a cross plane cranks allow for longer longer service, lives.
Most I-4s are flat plane to get even firing impulses.
I don't think I'd want to do flat plane for I-5 - it would shake itself apart, same for I-6. A V-6 flat plane would have uneven firing.
The whole point of a v-12 is to use the cross plane crank to eliminate both primary and secondary vibration issues, so wouldn't do it there either.
Just remembered: There is/was a Yamaha I-4 bike with cross plane crank. There are advantages to duration without power on a bike as it allows the rear wheel to hook up better. Harley's have similar setup but for different and now historic reasons.