Sometimes it works great, other times not. It's hard to say, and everyone has different experiences with it. Personally, it's better for me now than it used to be since updates, but it's still not perfect. A lot of people, myself included, go to the actual
MDE portion of the Disney website on our phones' internet browser.
It's the exact same thing, except you're going through the website as opposed to an app. A lot of us have found that to work better. There are also FP kiosks to use in the park to make FP changes. That's not ideal of course, but it is an option. Don't stress too much over it. The app being down will only ruin your vacation if you let it.