All a sentry farm is, is teamwork made simpler by the nature of Engineer's loadout. Fact is, all it really consists of is multiple people attacking/defending together; it just so happens that they're doing it with a static sentry setup instead of actively running around and aiming. While they're more monolithic and may be more intimidating at first, make no mistake that they're stationary, which is where group tactics and teamwork comes in.
Find me a team of good players in varied classes working in sync and I'll show you an infinitely more dangerous group of players than a bunch of engineers turtling behind a farm.
Find me a team of good players in varied classes working in sync and I'll show you an infinitely more dangerous group of players than a bunch of engineers turtling behind a farm.