There was recently a thread on the Ohioans for Concealed Carry discussion board had a post asking "what if your gun falls out in public".
A surprising number of people admitted to having their guns fall out in public.
The overwhelming majority were carrying in an Uncle Mike's or DeSantis cloth holster. I had a gun fall when getting out of the car to get gas--in one of those nylon DeSantis holsters. Last time I ever used a nylon belt holster. In my case the entire holster came unhooked from my belt and skittered under my car.
There are two problems with these particular holsters. The first is the relatively narrow clips used--Most narrow clips can easily twist off the belt when used alone, even if straight tugging feels secure. The same clips used in pairs can be very secure, since they can't twist without one of the clips coming undone first.
The second is retention. A soft nylon holster can't hold a gun securely without some sort of assistance, generally a thumb break strap. (A thumb break is a strap with a snap and a stiff section that can be pushed with the thumb while drawing to break the snap open)
A cloth holster with a thumb break and belt slots instead of clips is OK--probably not as comfortable or concealable as a good leather holster, but not likely to dump the gun. Nylon is also fine as a pocket holster, where lack of retention is a feature rather than a bug.
Although there is a bit of snobbery in recommending against nylon holsters, there are also a lot of practical reasons--Although it is possible to make a bad holster out of leather (I know, I've done it myself...) it is much more difficult to make a good holster out of nylon.