Fitbit and Garmin do not share their data with Apple Health which means Symple can not retrieve your Fitbit and Garmin data.
Garmin claims to connect, but lots of bug reports on various forums suggest there is either a bug or a big problem with the user interface.
There's fierce competition between Apple Watch, Fitbit, Jawbone, Misfit, Garmin and many other activity trackers. Some manufacturers jumped into Apple Health early; others didn't. But if activity trackers are going to stay in the public conscience, we need to make that data available and useful to apps beyond their proprietary ecosystems.