According to Wikipedia, an ISO file is (emphasis mine):
ISO disc images are uncompressed and do not use a particular container format; they are a sector-by-sector copy of the data on an optical disc, stored inside a binary file. ISO images are expected to contain the binary image of an optical media file system (usually ISO 9660 and its extensions or UDF), including the data in its files in binary format, copied exactly as they were stored on the disc. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created.
When I use
dd or some other low-level utility to write the raw ISO bytes to a USB stick I can't boot from the USB stick. Why is that? The raw filesystem structures are present on the USB stick. Sure, UDF or other filesystems are not ideal for usb but they should work, right?
I have tried this with the newer images of Windows 10 and Ubuntu 16.