Weirdly I never ever get framerate drops outside, but I've had a few on occasion inside when there was lots of dynamic lighting action going on.
While I'm thinking about it, here's a great article about Bethesda's studio culture and the tradeoff between making games with the scope that they do vs the lack of polish: http://www.wired.com/2015/11/fallout-4-bugs/
I keep seeing people comparing FO4 to the Witcher 3, and aside from W3 having 2.5 times as many people working on it, it also doesn't allow nearly the player freedom that the newer Bethesda games do. W3 is an action RPG in an open world; Bethesda used to make those (see TES 1-3) but have moved on to making worlds with some RPG bits thrown in there. And unlike other open world games, I can actually put a pile of time into a Bethsoft game (or Fallout NV) without being bored out of my mind.