It is supposed to be safe. It is still a good idea to check out Google's content guidelines, since there are some types of content that are incompatible with AdSense. In case such content is published, HubPages may disable ads after being alerted by automated content filters. But in the end, it is each Hubber's responsibility to respect Google's guidelines. Ads can be disabled manually for specific hubs, as long as they are not "commercial," meaning no outbound links.