A power outage can of course always result in losing any data not yet committed to non-volatile storage. Where filesystems differ is that some ensure that the on-disk data is always consistent even if a write is interrupted. This means that in the event of a power loss, everything successfully written is guaranteed to be intact. Examples of such filesystems are ext4, XFS, and NTFS. With other filesystems, power loss can result in an inconsistent state rendering existing files inaccessible. FAT, exFAT, and ext2 all belong to this category.
Not quite.
If power is removed during a write, then no filesystem can ensure the data is written successfully as.... well... the power has now gone....
This is why in enterprise environments, we have RAID controller cards with battery backed RAM.
Data is stored in RAM and written to disk, then cleared from RAM on a successful write.
If there is a power cut (either during a write or not) - the (uncommitted) data is retained in RAM (powered by the battery) and then written to disk once power is restored.
EDIT: Re-read your post (about 5x) and you said: "
This means that in the event of a power loss, everything successfully written is guaranteed to be intact." - this of course assumes that any writes took place
before the power failed. But no filesystem can protect against write failure / corruption when the power fails during a write procedure.