Kevin- Most people are going to brag about the best deal they did and maybe even exaggerate a bit. Sure 20%+ returns are possible but not necessarily the norm. You are well advised to consider ALL of the costs that go in, I can't tell you how many times I see ads that "forget" to consider many of those costs.
A very important consideration is the area, tenant and associated risk of the asset. A new build with a high income good credit tenant is not the same risk as a 50 year old home with a low credit blue collar tenant. The first may have a lower CoC return but might still be a better investment. Of course, measuring that risk is the delicate task of a great investor.
Finally, if you are worried about your CoC diminishing over time, you can always refi the property to return the leverage and restore the return. This is very typical in commercial properties (which usually have a balloon feature on the loan) but can be used in SFR as well. Happy investing.