Is there any disadvantage of buying a refurbished iPhone 6s/7?
It depends where you buy the iPhone. Generally it’s wrote when the battery is new.

On the Apple’s refurb, the batteries are new.

For the warranty, the websites offers 60/90 days because it’s not NEW iPhone, not because they’ll be decreasing shortly.
And the websites would tells you the level of the iPhone, if it gets scratch on the back, missing wire.