I have run several websites since 2009 and all of them have been ad revenue based. Only recently, due to a severe drop in ad values, I have taken one of my sites off of ad revenue ($0.10/1,000 views is not a good rate). Anyhow, there is a major problem with ad revenue being your only source of income.
Ads are greatly dependent on the content and the user viewing those ads.
Say Steemit used AdSense by Google (one of the many I have tried and can attest, is the highest paying for years running). If someone is invested in Google (uses the search, LIVES on Plus and clicks ads regularly, even using an Android phone with Google's operating system) then Google will give more weight to views by that person. That weight is placed against the value of the ad being seen and the weight of the site showing the ads (rumors have circulated that heavy clicks from sites can lower your percentage of the revenue from ads). This is great, if you have nothing but heavy Google centric visitors coming and have content that Google has high paying ads for that they give to those visitors.
Now, if you are writing about stuff that Google has no high paying ads for, no high value users visiting your site, and you have, say, a record of high clicks through ads then you can see hardly nothing from those banners and pop ups.
Other than annoying visitors.
Ads are the number one method to monetizing a website. Thankfully Steemit has another monetization option available and it works much better.
Rather than Google, or someone else, declaring the value of a visitor, or content, the members of Steemit do that. That is if Steemit would operate under revenue share or simply be like Facebook (and keep all the money).
I could see targeted ads working - those that are sold without the assistance of a third party "all or nothing" option like Google AdSense. This is harder and more time consuming, part of the reason most sites don't bother doing it. The rewards are much higher though.
The blockchain technology backing Steemit is a great idea and shows it can work - allowing ANYONE interested in owning a part of it to do so.