I've been carrying a 6906 for over ten years. The safety stays OFF at ALL TIMES. I use it as a decocker ONLY. It has never engaged itself in my holsters. Never.
Your 'instructor' was woefully ignorant of DA/SA pistols, IMHO. Given that the M9/Beretta 92 series has the same manual of arms, I wonder if your 'instructor' has actually fired anything except a Glock. And a few other Tupperware striker fired pistols.
And I should mention that I own Glock Model 19. I've never CC it, and it hasn't been to the range in 10 or 15 years. My 6906 and 5906 go with me to the range and get shot EVERY time I go to the range.
I fell on hard times back in the 1990's, and I had to sell most of my firearms. I sold ALL my 3rd gen S&W's EXCEPT my 6906 and my 5906. I really miss the ones that I had to sell, but I'm so glad I kept the 6906 and 5906. They are two of the finest 9mm ever produced, IMHO.
And you know: you just NEVER hear about '3rd gen S&W kabooms'. nor instances of '3rd gen S&W leg', either. Glocks OWN those two phenomenon.