I am going to let the bee out of my bonnet with a story. This is not related in any substantive way to any other current thread. But it, and some PM finally moved me off my butt and pursue the question....
It helps to have a wife who spent 30 years in the professional sewing business. So here is my story, confession, recommendation and bee release.
I asked my wife about the "modified french seam" and the "flat felled seam" question. The story is when she was working professionally in the costume shop for stage theater and ballet she used a flat felled seam in the appropriate situation. _HOWEVER_ her direct supervisor used what I have been calling the modified french seam. So... one technique has no substantial advantage over the other in terms of durability or strength.
In addition she said unless one is specifically trained in the use of the flat felled seam the "modified french seam" would likely to be easier to manage. This is particularly true for the less experienced DIY folks.
Wait for it....
However.... she also suggested that referring to the french seam model as a "flat felled seam (hybrid)" is not an accurate name and is a source of potential confusion.. Nor did she like the term "modified french seam". She objected to both on the same grounds. Since each is a defined style of seam, adding modifiers which are not clear can be a source of confusion. Hybrid flat felled seam is descriptive but not helpful as the "hybrid" is unclear. The same is true of "modified french seam" It is not the french seam which is being modified. (Yeah she's an author too, go figure.)
Her term of preference for clarity of communication is "Top Stitched French Seam" as that describes accurately what is being done so the established world of stitching and the DIY forums of HF are speaking the same language.
My recommendation is that we adopt for forum use the terminology that already exists in the outside world. That way the few clerks in Joannes who _do_ know something about sewing can be helpful to us. The world is larger than our forum.
However... for the sake of simplicity and clarity for folks who are beginning to learn the craft of gear making, I will stop waving my flag for the true flat felled seam. Even in the professional they are used interchangeably and one appears as good as the other. Lets deal with the one that is simpler.
The bee has flown away.