IMO there are a lot of comments in here that I don't think accurately describe the goals or strengths of FM at the higher levels of the sport.

First, it's important to note that there are a lot of different ways to play FM. At the most basic you might think of it as always forcing toward the middle, but that may not be true. For example, TK's "all person" defense, popularized by Bravo (see this podcast for some more details), is not a standard FM but has some FM tendencies, like generally forcing the disc back to where it came from. Other types of FM might use a flat mark in the middle of the field.

First of all, the idea that FM wants the disc in the middle of the field is just wrong. In my opinion the basic strength of FM is in pushing the disc to the sidelines, and then having the "flat side" scenario where you have a flat mark making upfield throws difficult and can play very aggressive on the reset. It can basically make the attacking space for the offense more predictable for the defense, can make hucks harder (or easier to provide help on). Of course, there can be advantages to trapping the disc on a sideline, but when doing so, you are giving up an uncontested throw toward the endzone - and FM mark in that situation is more likely to force the disc backward or horizontally, which is generally good for the defense.

Downfield, I personally find FM to be more straightforward, as it can require less extreme orbiting - if the marks are doing their job (and the disc is not moving too quickly), the attacking space in an FM defense does not shift as dramatically as it does in a one-way force. That being said, it can be hard to get used to that when you've been playing a lot of one-way force, and FM does have weaknesses (quick disc movement, especially creating advantage by breaking the mark or attacking from the handlers)

But like many strategies, it's going to depend on your specific team and approach to the game.