different brake line materials what do you use?

and don't say "pre bent". most of what I work on, there is no such option. I wind up making and flaring alot of my own lines. I do enough that I bought a Mastercool (brand) hydraulic crimper. I have taken a liking to the (usually "army green") coated steel line, since that came out. I have heard alot of talk about that "nicopp" line that bends so easy and flares easier/ I just bought my 1st roll of it since I am "out" here at home. usually 1 roll (25 feet) will do a whole truck or car, with maybe a few feet left over. (unless its one of the newer cars that need 2 lines run all the way front to back)
Doing lines front to back right now, on one of my own, replacing all the lines... front to back, and left to right from the master on out. (96 Dakota 4wd) as I had to put a master cylinder onto it/ and as I had my wife up in the truck pumping the pedal, while I bled the brakes/ I watched the line that goes front to back, blow.... right behind the nut where the front to back line, threads into the rear hose. it was absolutely bone dry when I put the truck up in the air.... like I said I actually saw it pop.

so I decided to try this Nicopp line, and so far I don't know if I like it or not. The easy-ness to work the line... straightening the roll and bending the line without my bender (both happen to be at work when I need them here, typical) and I can tell the flaring tool doesn't have to work as hard as with regular steel line. but, it pinches easily too which I do not like. means it will crush easy, if something kicks up from the road. or being a 4wd, something on the way to the campsite or fishing hole.... how likely is this soft tube to puncture?
I know that regular copper line has never been approved for use for brake line material.... what makes this stuff "OK" then?