There are actually already all the answers needed (with the exception made for the specific results of a direct test of the fbinst formatting to the specific machine).
fbinst is an exceptionally good tool
, that achieves almost (if not fully) 100% bootability on *any* system.
In order to achieve this exceptionally good results it goes - in several ways - beyond and well over "common standards".
A stick partiioned/formatted/prepared with fbinst is a VERY good solution
if - like in the case of WinsetupfromUSB it is an "install media", or if you prefer it is used in a "format once and forget about it" approach.
Still, since it goes out of the said "common standards" it is incompatible with a number of tools (particularly "Commercial" and "Easy to use" ones) related to partitioning/re-partitioning/etc. so it is NOT
a good-for-all solution.
As a matter of fact also installing the grub4dos grldr.mbr to the MBR (and following few "hidden sectors") is already out of these standards, and this is the reason why (besides the mentioned incompatibilities with some BIOS/Motherboards of the grub4dos MBR code) - normally - I don't suggest the installing of grldr.mbr, but rather using a "common" MBR code that simply loads the Active Primary partition.
The above mentioned "incompatibilities" are NOT however a problem for "people that know where their towel is", but the number of really hoopy, froody guys/gals is relatively limited, whilst the number of whiners
is exceptionally large, so the warning is nothing more than a preventive measure for the latter peeps, in order to be able when (and they will before or later) come whining because they lost some data or managed to - just after having made the "perfect" ultimate booting thingy - mess it up badly, to tell them:
STOP whining, you had been told it was dangerous!