Tor, or more specifically the Tor Browser Bundle is not some huge hurdle to overcome if you want to browse certain content. I imagine for the audience of Sci-Hub, downloading then using Tor is no biggie.
The only real hurdle may be discovery of the right .ONION but once you search for `The Hidden Wiki` on DuckDuckGo you will be soon in Onionland and there are plenty of search engines and directories on there that could potentially point to the latest Sci-Hub .ONION