If you can't handle Android Pie's bugs and/or performance issues, you can always downgrade back to Android Oreo. That said, downgrading is a major pain.
There's a very good chance you run into a bugs and performance problems while you're testing the Galaxy Note 9 beta. And it's possible these issues will force you to drop your device back down to official, stable software. If that happens, you'll have to wipe the data from your phone.
This is why backing up your data before installing Android Pie is crucial. If you don't, you could lose some of the data stored on your phone.
Backing up your data and restoring said data can take quite a bit of time and there's always a chance you encounter data loss when moving your Galaxy Note 9 from Android Pie to Android Oreo.
If you don't have time to deal with any of this, you'll want to avoid the beta and stay put on Oreo.