Leaving out the opinionated ranting for a while...
Most plastic pipe /push fit problems are due to miss-assembly.
Plastic pipe can burst under fault conditions where copper wouldn't.
It doesn't hold the same combination of pressure and temperature.
If the main boiler stat fails on
some boilers, the water can get
much higher than 100º before the high limit stat turns the boiler off.
The pressure in a sealed system can be up to 3 bar. Manufacturers can be a bit coy about giving the temperature at which the pipe will actually hold 3 bar!
If the High Limit stat fails as well, the water can get to 150+º (at 3 bar) before it boils. Plastic pipe certainly won't stand that.
This
happens. One Plastic pipe manufacturer told me they get about one complaint of a burst every week.
Modern boilers are much better controlled than some older ones (like Ideal Mexico's), so a failure of just the main boiler stat wouldn't make the water hot enough at the normal 1-1.5 bar to burst plastic pipe, but it's close.
Is a burst pipe a disaster - it depends. If you have a runaway boiler it could be a minor problem. Sooner or later something has to give.