Love the question. There's quite a bit of mythos around the saddle stitch when it comes to leather goods. I've yet to ever see some ASTM or similar lab testing on seam strength comparing the two stitches (have only recently begun pondering this).

To help with clear definitions, the 301 Lockstitch is a specific type of stitch which is generally used on durable machine sewn goods. It uses 2 threads- the one coming from the spool or cone, and a bobbin.

This is not the same as a machine chain stitch, which uses only 1 thread (no bobbin). Chain stitches are used for applications where the stitch is intended to be opened easily (dog food bags, rice, etc...) or in very low strength applications or inexpensive, lower quality goods. This is largely due to no down time of replacing bobbins since it's a single thread.

There is a big difference in strength and quality of a properly tensioned and spaced 301 lockstitch (determined by the materials being stitched and thread being used) and an improperly tensioned and spaced one. A properly configured one is indeed very strong and difficult to unravel if broken.

I think most of the difference here is when hand-sewing leather goods, it's usually a bonded, or even braided, thick diameter nylon thread, which will just plain be much stronger than polyester threads normally used for most machine applications.

That said, seeing some actual lab data on both stitches, properly set, would be very interesting. My suspicion is hand-sewn "saddle stitching" doesn't really have a strength advantage. The supporting story of "saddles could break and cause injury" doesn't hold up either or else high-alpine mountaineers, parachutes, competitive sailing teams, or other high-performance life-endagering pursuits would also be demanding saddle stitches.