In my experience, Google Now finds my flights in my Gmail 100% of the time. For at least a dozen flights across the US. And for some airports (SFO is one, unsurprisingly) it even has the terminal and gate information ready! Google Now makes the "arriving at the airport" experience better every time for me.
In order for me to depend on Now, I need to know I can trust it and it only got one of the flights right. There is no way to know if it will do the right thing in the future unless you keep checking and having a backup plan.
More accurately it is giving you a random subset of information that might be relevant right now. The behaviour is not deterministic so you can't rely on it.
It's sort of scary to think that they're now going to parse out and generate non-email records of potentially confidential information such as your physical location at a point in time (like a flight number + date).
I steal private data for a living and that's just fucking creepy.