Obviously not! As the site's staff keep reminding everyone, the DVOA of any single player does not represent a judgment on the player, but simply gauges his performance. For example, it's a well-known fact that OL blocking is key to an RB's DVOA, so that an RB's DVOA is actually a reflection of the performance of the entire team as far as running the ball. The difference with RBs is that you can compare two or more backs from the same team and DVOA will tell you how good each is relative to the other.
Applied to Tannehill, every staffer and the usual posters -very smart and knowledgeable people, most of them- know a QB doesn't play on an island, that the OL and the receivers and RBs all have a role to play in explaining the DVOA of a QB. Take Mike Tanier, a former staffer at FO and currently at Sports on Earth. Last week he wrote a piece about Ireland and in passing explained what most everyone knows, that if anything Tannehill has been doing pretty darn well so far despite his bad DVOA.
I also think FO it's a great place to start learning how to apply statistics correctly to the game of football, as there are many caveats that need to be considered, and you can't just yield stats and correlations like a club and expect to derive something meaningful.
As far as the 3rd and long example, on the other hand, no, the fact that you face more of those situations is not of particular interest. Every QB is rated according to a baseline, that is, what the "replacement level" QB would do on 3rd long (which is not much). In other words, the bar isn't very high in those situations, since you're not expected to convert very often.
