Filesystems are separated but i used to bind mount Folders like Documents, Downloads, Pictures etc.. into the Android container.. thereby sharing the important data.

Other integrations could include making sure your contacts between Android and Sailfish are in sync.

Not sure what the status of notifications is.

Keeping the clipboards in sync.

Half of these things can be "hacked into" by using something like kde connect in the waydroid container.

Integration with Android intents like share etc .. would be stretching it..

It'd still be nice to do it properly.