Most of the free web hosting providers and free blog hosting providers do have restrictions on the websites and blogs you have with them :o
The restrictions are not only limited to displaying ads on the blogs or websites but there are certain limitations on the content you can have on the websites and blogs you have with them :( No doubt, there are certain restrictions about content when you host your blogs or websites with paid hosting providers also but these restrictions are limited to some extent with satisfactory reasons.
Also, if you choose free blog hosting providers or free web hosting providers to host your website, it might be possible that they will place ads on your blogs or websites once your website or blogs have good content on it
Also, it might be possible that they will remove your blog or website if you do not update it.
You cannot expect good quality service if you choose a free web hosting provider or a free blog hosting provider which is certainly not good as you might need assistance at any point of time to host your website or blog :(