Hey, my two cents, FWIW: Sharkbite and similar type of push on fittings seal the fitting against water leakage with, basically, neoprene O-Ring and seat. They hold the tubing in the fitting by means of "the shark bite" or a sharp stainless steel collar that digs into the tubing. (Note: removal of the fitting has a small tool that presses on the top end of the stainless steel collar, spreading the teeth and allowing you to pull the tube out)
I've had good success with Sharkbite, and Blue Hawk fittings. Less so, for some reason, with the Watts equivalent--where I had a number of them fail on me and when replaced with Blue Hawks, everything was fine.
Just like in order to make a very nice, long lasting solder joint in copper tubing, you need clean fittings, the right amount of heat, solder and flux, there are standards you must follow with these push on fittings. First the tubing end must be square, properly cut with a tubing cutter. If you cut it on even the slightest angle with a hacksaw or equivalent, you are setting yourself up for trouble. Second, the tubing end must be clean; I treat the tubing ends AS IF they were going to be soldered. Clean with fine steel wool, and wipe down after. Lastly you need to mark the tubing for the proper insertion depth, and ream the tube edge. They make a simple cheap tool for that.
If any of these things are not right, you can set up for immediate failure (fitting won't seat or work at all) or for failure in the near term. I've had both happen and have learned my lessons.
Neoprene doesn't last forever; when exposed to ozone, sunlight, corrosive chemicals, etc. it can deteriorate rather quickly. But potable, domestic water for the most part is pretty good and free of most of the things that cause it to fail. Thin wall tubing will certainly deteriorate faster than the thicker brass casting of a Sharkbite fitting, so if the water is corrosive enough to cause pinhole failures, the fitting may remain working.
I should also add that copper tubing for plumbing is for the most part, an element not a compound. It must be manufactured to a 99.9% purity to ASTM standards. It does not get brittle with age itself. It can get "work hardened" but boy if you had copper tubing in a plumbing system that moves so much as to be affected by work hardening, I'm here to tell you that you've got a whole lot of other issues to worry about. Plastics do get brittle with age as they "outgas" over time, and the plasticizers used to keep them flexible. I'm sure everyone has seen all kinds of plastic that have aged and gotten brittle, in and out of the plumbing industry. That is the ONE THING that kind of scares me about PEX tubing and plastic push on fittings used to join them. I don't know if it has a name, but "age embrittlement" is a possibility.