OEM can rust from the inside out. Granted, if the brake fluid is changed (flushed) every few years, it will never happen.
I have seen two brake lines that looked perfect from the outside, rusted through from the inside resulting in brake failure. One was a first gen Camaro.
Again, not likely, but possible. I had to replace my original fuel line because someone had cut off both ends. Went with stainless. Figured it would be a good time to do brake lines as well. I capped and saved all my original lines. I had no sealing issues. Yes, it is harder to bend.