The Werth contract was a little overpriced, but it would be a mistake to term it a bad deal for the Nats. He gave them 2 legitimately great years. The first couple of years he was a little disappointing, but was still a positive WAR player. The last 2 years have seen him slip defensively but that is the price you pay for a long term deal. One could argue that he gave them credibility, filled a position of need, and that they had the money to spend on something.
Also, I have not seen much evidence that long term deals for power pitchers have fared worse than deals for guys that throw less hard. Often the power pitchers can still win if they throw 93 instead of 96. The non power guys can have trouble slipping from 92 to 89. There are lots of comps for Strasburg that work out if you look at performance in age 28-34 seasons. Sabathia and Verlander for instance look like cautionary tales now, but they would have been fine under the Strasburg contract for age 28-34. Here is the WAR by year from age 28-34:
Sabathia: 6.2, 4.6, 7.5, 3.5, 0.3, -0.6, 1.0 = 22.5 total
Verlander: 8.4, 7.8, 4.6, 1.1, 2.2, -0.1, TBD = 24.0 total so far
Each WAR is worth somewhere between $8 and $9 million today. Over the next 7 years it would be conservative to say they would average $10 million. The chances of Strasburg earning 17-18 WAR over the course of 7 years is pretty good. It does not matter if he is injured and ineffective for 2 years if he is really good for 3 or 4. And the chances of that happening from ages 28-30 are pretty good.