There was one practice I avoided in past years, building Mini Sites. Actually I did nothing in the past years but thought it was fishy or itchy to do that. For me, something was not right with them. So I stayed away from them. My point was, what happens to the efforts and resources you allocate into it when the thing is busted? The sites were later slapped by Google for piling up garbage in the Holy Searchland. Mostly in the form of Thin Affiliate Sites and in many other worst forms. The most irritating post I saw on the practice by was from a well renowned blogger domainer Elliot. Not only he kept bragging it but also endorsed it. Many built hundres of sites and though earned thousands, I am sure, this year or the other, thousand such sites will be dumped. Many even lost their Adsense accounts. Thus I name it the "Grand Mini Site Bubble". Have anyone else named the phenomenon already?
if you build them right you will have no issues and can make a nice income. you need to know how to build them and not waste time.
Original content and decent SEO structure is king. Mini-sites are fine , even for affiliate marketing, you just need to manage them well and provide relevant original content. You also need to keep working at getting an audience for the mini sites, whether it be directories, link trading etc.. They take effort initially but once they're rolling you can move on to others and eventually hire help to keep the machine going.