The rear brake will lock the wheel effortlessly with a ss line.
An Extreme Pro isn't available for the rear, and I'm unsure if any ceramic or carbon pad is.
Either way, with any upgraded pad the rear will lock even easier.
I ride alot of standup wheelies, when trieing to ride very slow ones the rear brake helps. With the ss line I had on my gen2 it made the rear brake too responsive for that(I had to get used to how twitchy it was). It did make it much easier to start a drift though.
For normal riding, with minimal rear brake it's not an issue...unless you're unexperienced in proper braking.
It's so easy to lock the rear up that it could get you in trouble if you don't know how to handle it.
The ss rear line does look better than stock though
the one I had was free during a sale when I bought the front lines, otherwise I wouldn't have bought it.
Stock or ss rear line is fine, they just feel and respond differently. I wouldn't be concerned that any bike did or didn't have either. It's a personal preference.