Yeah, the main thing that makes sentries overpowered is the fact that other enemies are always about. A level 3 sentry on its own (with or without an engy), is no match for a demoman, soldier, or if its close, a heavy. Without an Engy they're childs play to destroy. As any class.
However, like you say you never get the chance to focus on the just the sentry itself usually. An uber is just there to give you protection from everyone else while you take care of it.
However, like you say you never get the chance to focus on the just the sentry itself usually. An uber is just there to give you protection from everyone else while you take care of it.
