There's absolutely a chance you could be/get pregnant.
You can take it out up to 3 hours without having to worry, but anymore, and the chances go up..
As for comparing NuvaRing to bcp's, it's pointless, they're different things, and it's silly to say 'this is how it is with bcp's, so I guess it's the same for everything else', kwim?
If it were me, (and I have btdt, with this), I'd worry, but more likely than not, everything's fine.
I also have to remove mine (only during specific positions though), and it has slipped out on me once (and I did not know for at least a couple of days!).. It's okay, and safe, just something you do need to remember about.