If you played exactly by the books, no, it's not broken. Now, you'd have to start them at level 1 with no exp, and they'd have to progress at the normal rate (some 13.3 encounters per level, I hear?). By the time they got to the levels where things become problematic, they should know what's up and how to handle it, assuming they read the core books cover to cover.
At level 1, everybody is fine.
At level 3, you start fighting things like dire wolves and ogres, where people clearly start to see how they can be shafted by the rules if they don't play smart and work together.
At level 7, you get things like behirs and hydras, which will absolutely murder one pc per round unless they do everything right (and have strong, focused builds or otherwise get really lucky). But by level 7, they've been through some 75 odd encounters and have been playing for months.
It's harsh, and has bad options they can fall into, but it gives them a way out, as long as they're smart and tactical-minded (and not too proud to change when they're wrong).
I honestly like it just the way it is.
Can't speak for PF though.