It is really early in the process for us players to give proper feedback, but since this is a very similar mechanic to one used in the 5th edition of a certain-game-that-shall-remain-nameless...:
This is about the boxed text from page 20:
FAVOURED VS ILL- FAVOURED: It is possible for
a roll to be both Favoured and Ill- favoured for
different, conflicting reasons — when this hap-
pens, the roll is resolved normally (roll one Feat
Die only), even if multiple sources would make
a roll Favoured and only one would make it Ill-
favoured (or vice versa).
We (me and almost all players who I've played with) have always felt that, when multiple sources can favourably or unfavourable influence a PC's roll, the fact that they all cancel out really ruins the immersion.
"[...] OK Emily, your character has a magic artefact, you have surprised the sentinel, and the sentinel is half asleep from boredom... But the lighting is poor, so that cancels everything else out: make a normal roll to see if you succeed [...]"
Blargh!
If there are mechanical balance issues, I'd understand; but besides the fact that there would be more favoured- and ill-favoured rolls if considering the respective number of sources, I don't see what the issue would really be...
Have you considered and rejected taking into account how many sources, or was it decided right away to go with the "normal resolution" rule because players would already be familiar with the concept?