Google has told us for years that content hidden in tabs, accordions and so forth are not given full weight and may be ignored by their ranking algorithms. But that is to change with the mobile-first index - or at least it appears it might change.
We actually saw this coming when Google AMP released accordion support for AMP.
But now, Gary Illyes from Google said on Twitter in response to many folks using this as a feature for user experience purposes to make content manageable on mobile that Google will likely give this content full weight.
Here is the Q&A on that piece:
@schachin no, in the mobile-first world content hidden for ux should have full weight— Gary Illyes (@methode) November 5, 2016
As you can see, Gary said "no, in the mobile-first world content hidden for ux should have full weight."
Will this also apply to desktop only content? Hard to say - but I would not assume so.
Forum discussion at Twitter.