i can name three places where sentries can be abused
1.hills (the most known one)
2.inside pipes (where spies can't sap shit)
3.inside the other's spawn (by buying noclip and placing sentires and shit down)
what i was suggesting is that you rephrase what you said because i garantee every map has a place for "gain an unfair advantage" it's called a choke point.
Shitmas, I've played on every map at one point or another, and they are pretty well balanced. Also, in two of your examples, the sentry can still get destroyed by other players through regular means. And in our maps, it's actually easy to clear a choke point. So obviously, your first point is the one that WILL get you kicked if you fail to comply when an admin tells you to not build in there.
And what you need to do, is quit making it seem like I don't know what I'm talking about, because my instructions are very clear, even provided examples.