First and foremost, I know how talented and durable Prince Fielder has been over the last five years. He's a career .282 hitter, which is great for a power hitting first baseman (at least nowadays). I'd be surprised if that average isn't higher than .300 again, as he will be hitting right behind one of the most gifted hitters in the MLB, Miguel Cabrera. The Tiger offense probably has the best 3 and 4 hitters in the league, combined with a lot of speed at the top of the order and capable hitters to surround the sluggers. The offense will be great, no doubt, but the defense is the biggest problem.
Miguel Cabrera has agreed to move to 3rd base and the Tigers expect him to play there this year, making for one of the worst defensive teams in the league. Cabrera last played 3rd in 2007 and defensive metrics had him as the 17th of 21 qualified 3rd baseman. Furthermore, Cabrera has gained about 40 pounds since he last played 3rd base, limiting his range by a lot. It's going to be hard watching him play there. Additionally, Prince Fielder is not exactly going to be fighting for the Gold Glove either. Prince Fielder has cost his team the most runs among 1B since 2006 (48 runs), which is ten more than any other 1B in the MLB. Of course this stat is not the final determination on how good/bad of a first baseman he is, but he's not good that's all that matters. Of course the Tigers will be able to DH one of these guys, but rumors have Delmon Young being the opening day Designated Hitter.
An already mediocre defense will get a lot worse this year. Yes this deal will help them score more runs in the next few years, but the defense will start getting atrocious and hard to watch. The pitching rotation won't get a lot of help from their defense, especially in 2020, when Prince Fielder will be owed another $25 million at the age of 35.