I have only had VERY limited experience with French Flints and that only in my Brown Bess Lock. My limited experience with the French Flints agrees with your experience and I prefer English Flints for their longer life as well.
However and this information is probably only good for those wanting to be more HC/PC (such as for living history, etc.), the French Amber Flints are much more HC/PC for those using civilian guns before and during the AWI. Please allow me to explain.
T.C. Hamilton in his book "Colonial Frontier Guns" and Dr. De Witt Bailey in his works on 18th century British Military Guns and some German made rifles, both point out that the SHAPE of gun flints used by the British Flint Knappers did not begin to resemble what we now know as "English" Flints until about 1775 at the earliest. Up until then it was a exceptionally well guarded "Trade Secret" of French Flint Knappers.
I am referring to a flint that has a sharp edge in front, a less angled/sharp back edge and the more or less flat top. Prior to 1775 at the earliest, British Flint Knappers did not know how to make that shape of flints and were still knapping "Gunspall" or Wedge Shaped Flints. It was a HUGELY important break through when British Flint Knappers first offered that then "new" shape to British Ordnance in late 1775, that French Flint Knappers had been making for many decades. British Ordnance had long known of the superior shape of the French Flints and actually purchased HUGE quantities of such shaped flints from France in between wars with France and through the Germans or Dutch when they could during War time.
Now, the Superior Shaped French Flints came into America to the English Colonies very early in the 18th century and were highly prized by American Colonists vs the early English "Gunspall" or Wedge Shaped Flints. These flints seem to have come in mostly by Dutch Traders, whether the French Flints were smuggled into the Colonies or the extra Tariff Tax was paid on them to come through normal channels.
The reason I mention this is some folks may enjoy adding an special added HC/PC bit of authenticity to either their Pre-AWI civilian arms or even their AWI Militia or Military impressions to be either shown or used in front of the public, by using the French Amber flints.
Gus