Nylon is a very strong material. All nylon clevises are self-threading which is a good thing. Any given size threaded rod can vary significantly between manufacturers. If you thread the clevis onto the actual threaded rod it will be used on it will be a good fit, it will not slide off, it won't unthread itself and it doesn't conduct electricity.
Do not even think about gluing a slip-fit nylon clevis to a pushrod. Ok, I brought it up and now you've thought about it. That is because I've seen it done. Don't ask...
Threaded metal clevises lose on all the above points. You get what you get and it fits the rod however it fits — sometimes good and sometimes not good. Metal clevises can destroy the soft threads on the rod from vibration through the airframe over time. The looser the fit, the worse the problem.
Some pushrod types can conduct electricity and radio waves. This can be a problem if it is not insulated before it gets close enough to the servo to feed back into the radio system. If a pushrod or pull-pull system is not conductive then it doesn't matter what type of clevis is used from conductivity stand-point.
However, if the pushrod is conductive, then I use some type of connector that is not conductive to help insulate the radio. That usually means a nylon clevis.
The two drawbacks to nylon are its flexibility and wear properties. The pin in a nylon clevis can flex and add more play in the system. This isn't usually a problem if the appropriate size clevis is used and if it is fully closed and locked.
A worse problem is that the pin can wear through and eventually break. I've had planes that had many seasons of flight on them that had clevises with the pin nearly worn through. I don't make it a habit to open clevises and inspect the pin, so this sort of thing could lead to clevis failure followed by destruction of the model.
Nylon clevis pins wear fastest when they are inserted through metal, such as a throttle arm, or an abrasive material, such as fiberglass-filled nylon arms or horns. They wear slowest when inserted through a smooth, non-abrasive bearing surface such as another piece of nylon.
I'm not trying to frighten you away from using nylon clevises. I use them all the time and trust them. But the pins should be checked occasionally for wear and the clevis should be replaced as necessary.