Who would be crazy enough to build a business around an ad service that can yank the plug at any time without any prior notice?
Personally, I hate the idea of not having control over my income.
Adsense is a good complimentary source of income, but building adsense only websites is a dead end road.
This is exactly the problem with adsense making money strategy! Your entire operation relies on Google.
Imagine if they make a change that has a significant negative impact on your sites, or if you made one wrong move and somehow got penalized even when you're not at fault, it's game over! You don't have any control over their process and Google doesn't care about you since your sites are not real companies or anything. Suddenly all that hard work you put in during the course of 5-6 months is now down the drain. What are you gonna do? Spend another 5-6 months to build more sites? Not to mention new competitors may pop up and try to outrank you on your keywords or get your sites sandboxed.
Adsense is too unpredictable and unreliable in my opinion, it's like a never-ending guessing game that you're forever a chaser. If it requires only minimal effort then I guess it would be okay but that is not the case.