I'm ok with frame rate dropping here and there. Constant frame rate issues, controls that aren't fine tuned and out right bugs that crash or hang the game are my definition for "broken". Since that's what's being reported here about Firewatch, it's broken in my books.
Case in point: Just Cause 3 has performance problems on the PS4. Some have been patched after release, others not. However, there were no reports of the game crashing or hanging. I didn't see anything bad reported about the controls. In general, the PS4 frame rate and loading issues weren't as bad as the Xbox One release. When I finally got a chance to play it, I was OK with the issues. However, I don't think that game is broken. It's less enjoyable and I don't think it's worth the full price because of the issues - sure. But, broken it is not.