Sound Change Susceptibility

I was looking at a chart of reflexes of PIE and Proto-Semitic consonants the
other day, and noted that while, say, the ancestral velar stops are mangled
wantonly in many daughter languages, *m and *n are perserved in every language
listed (in initial position at least).

Now, is this just a quirk of these particular families, or do different sound
have differing "intrinsic" probabilities for changing?

                                                              Andreas

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