I cannot seem to find an official source for it, but it appears that "UDF" if copied verbatim, cannot be booted from USB. And many ISO files are UDF formatted.
Appears that for "cd rom drives" being able to boot from them is accomplished by their conforming to the El Torito standard.
However for "normal" disk to boot, the "MBR" has to be set in it (which is apparently different bits than the El Torito bits). And apparently you can set one or the other, or both.
So basically, if it's an ISO that's "just" El Torito, you "have" to use a tool (or carefully do the formatting, partition marking, and file copying manually), that will give you a MBR formatted, bootable USB. Copying raw ISO bits is not enough.
A "few" ISO's may not need any special tool: (note the mention of "isohybrid," whose documentation mentions that normal ISO's (UDF formatted) are only bootable from CD drives unless you add an extra master boot record [i.e. outside of UDF spec], apparently many main-line linux distro ISO's are made this special way, but I know from experience windows install ISO's of at least windows 10 are not).
For the ones that do need it, apparently you should avoid raw disk copies from ISO to USB (ex: the
dd command on unix*'s). Or you'll run into the problem I did.
In my experience, typically you'll be converting from a UDF ISO to a FAT32 USB device. (The former is El torito, the latter MBR, for their "boot system").
How do I place a bootable ISO on a USB drive? (lists lots of utilities that do this "for you")
See also https://apple.stackexchange.com/a/234290/25085 for OS X related options.
https://blogs.msdn.microsoft.com/matt-harrington/2012/03/27/how-to-create-a-bootable-usb-drive-from-an-iso-image-for-uefi-systems/ (mentions that it needs to be FAT32, and have files copied from the ISO).
No amount of unetbootin seemed to work for me, bootcamp neither on newer iso's. I'm guessing I was installing to an older box which couldn't read newere boot schemes. Using rufus on windows "MBR" and "NTFS" (fat32 might have worked as well, but got some complaints of "install.win" being too big) and it worked for a windows 10 iso.