This is probably the biggest flaw that I see with these web-mined coins- there'll almost certainly be at least adblockers or something of the sort blocking out these web miners as soon as they start popping up widely on sites, and the subject of crypto overall is still relatively unknown as of right now, though adoption is helping the news get out more about crypto. I can only really see this being adopted on sites that are already targeting crypto-browsing users as well.