Overclocking on air is fine - air coolers almost always outperform AiO coolers at the same price point, so it's only above maybe $125-150 that you really see them even starting to break even.
Why? Because when it comes to cooling, what matters is surface area. Circulating water is just one way to move heat from the CPU to the cooling surface, and heatpipes are really just as good at it, and a lot cheaper than a pump, tubing, coolant, hose clamps and a waterblock. For an AiO at the same price as an air cooler, it will almost certainly have less cooling surface because of the added costs of those components.