This situation happens most often when an organization has multiple AdWords accounts that all create campaigns for the same website. A GA account can only be associated with a single Google AdWords account. If a campaign that generates the visit belongs to an AdWords account which isn’t linked to the given GA account, the visit will show up with the keyword “not set”.
No, unfortunately we can not guarantee that the absolute safety of Google Adsense Traffic. This is because we have seen instances in which websites were penalized for receiving large bulks of traffic (and clicks) to their Google Adsense ads. Therefore, if you use it, we strongly recommend you to use it in moderation and in conjunction with other traffic sources. More information >>