That it does not allow for ads in between blocks of content, so that you can still have your own block-based website design, as well as keeping track with analytics or other features from outside software such as Google Analytics (or whatever).
There are some ad problems when using certain plugins/ad networks - I haven't been able resolve them myself yet though. It's worth trying! We used this product during our transition period to try out different platforms before deciding what we wanted going forward once things were up and running again at full capacity.