Usually for me it's unreliable only when line of sight from phone to car is blocked. That's with an iPhone. I hear Android devices have more issues.

Edit: And the new fob is awesome. Different tech from the old one as you've probably heard, with different security/convenience tradeoffs. (With the old one being more secure, but to an insane level, and the new one being ever so slightly less secure but still very good in my admittedly fuzzy understanding of it). No issues whatsoever since getting that.