I always prefer to create my VHDs (fixed or expandable) directly with Windows Disk Management, of course the two tools wimb mentioned also make use of it.
In order to experiment the full capavilities and limitations of FFU I just ran a few more tests:
Using same 10x64-WB.wim and a 15 GB expandable VHD, MBR with tree partitions as following:
First Primary Partition FAT-32 Active: Boot: used 9.57 MB of 512 MB
Second Primary Partition: NTFS Windows: used 7.24 GB of 10 GB
Third Primary Partition: Documents: used 908 MB of 4.5 GB
Installed Win to second partition by means of DismMountService_191116 (Dism 10.0.17763.1): 7.24 GB >>> 268 sec.
Installed all required files/folders on Boot partition, and copied some files/folders to Documents partition just to better simulate an usuall HD.
Then captured the 10x64-WB.vhd (8.136 GB used) as Image.ffu on 87 sec. FFU size: 5.05 GB
Created a new 15 GB 10x64-WB-2.vhd only inicialized but not formated and applied the FFU on 193 sec. (193/268 = 0.72 or 28 % faster than WIM).
Then I mounted the FFU with DMS to a folder on my HD and only thing visible is Windows partition, the other two partitions do not appear (Yes, I have active since the source Wim and also on the running OS show all Hiden files, folders and drives and also unactivated Hide Protected files of OS), but on Windows Disk Management it is possible to assing a letter to each one of the partitions of the mounted FFU and this way all can be seen, good if we need to extract only some files/folders from a partition to a location on our HD, from the Mount folder only Windows remain visible, then removed the assigned letters and dismounted the FFU without making changes.
I though as a remote possibility that maybe if a partition do not exists during capture it may be possible it will not be erased during apply the FFU.
To test this I deleted Documents partition on 10x64-WB.vhd and recaptured the VHD as Image-2.ffu, dismounted it and mounted 10x64-WB-2.vhd and formated Boot and Windows partitions without touching Documents partition and re-applied the new Image-2.ffu, and when finished the 10x64-WB-2.vhd only has the Boot and Windows partitions and Documents partition dissapeared, there is a free unformated space corresponding to Documents partition location.
Well, only thing I can say is I'm agree with wimb: WIM files are more versatile and usefull than FFU files.
But also I can say: I tested all alternatives that came to my mind and I keep no doubts now about how FFU works.