Well, you definitely shall backup before resizing filesystems with valuable data on them.
And better double check that there is sufficient free space for the single shrinking action (the first resize2fs of the “home” volume), which is implicitly provided by a freshly flashed SailfishOS.
The shrinking of the “home” volume is the only really critical action, which may result in data loss. Note, that it should abort with an error message without doing anything, if there is insufficient space; but if it really fails the “home” filesystem might be spoiled.