"I think the french seam is just as strong as the others"
with French seam, it is in peel - the thread in the seam takes all the strain
in a flat felled seam, there is also some friction between the two pieces of fabric
also, in French seam, all the strain is on one row of stitches, then it breaks, then all the weight is on the second row of stitches. With flat felled seam, the two rows of stitches share the load so are less likely to break
I use french seam all the time, but it is a little less strong
with french seam you only see the joint between the two pieces of fabric, and maybe some of the thread in the seam. That may be cosmetically more pleasing.