In the past, the typical reason why certain Dim Mak points worked was based on "chi", the disruption and manipulation thereof.
Times change, science changes and as such, we began to understand the body better.
What was "chi", now became many other things such as automatic nerve responses, the body's own defence mechanisims working against it, nerve damage, internal damage to organs, etc,e tc.
The main issue many have with Dim Mak is its apparent uselessness in an actual fight, outside the typical points that are common in every system of H2H combat.
We must remember that Dim Mak was not only about empty hand attacks, but attacks with weapons, including specialized weapons such as needles.
Many of the DM points work ver well with a short stick or sharp object, less so with your empty hands.
Unless of course your hands have been uber forged.
Dim mak is, like IP and IV and all that, a subsystem of H2H fighting and not some stand alone "magic bullet".
In other words, the better fighter still has the edge, no matter what.