It's not the pressure, it may be more to do with the corrosiveness of the local water.
Though I've found that fittings will spring leaks too.
I just ran new plumbing for my bathroom using tye M copper 3/4 and 1/2.
A plumber just told we type M is not code for my area and I should have used type L. He also said not to freak out because type M is ok.
After research I see that M is code for NY but not in my town. Also pressure rating is 700psi for 200 degrees as copared to 1000psi for type L.
Does it really matter for a residential system operating at 80psi?
that was my next question lookstome like the standard fittings are of the same thickness of type M. Are there different thicknesses on fittings?
No. And if the water is going to attack type "M", it will do it with "L" also, it will just take a little longer to start leaking.