The insulated underground pipe is in a plastic tube.. a seamless plastic tube.. so even if it was burried in mud, and the mud froze,, there would be no damage to the pex pipes inside... and yes, nomatter how good your insulation is, it is just an 'R' factor, a resistance to heat travel... so some heat always travels through insulation. The ground around the pipes usually stays frost free, but only for a few inches around the burried pipe.
The amount of heat lost to the ground through the insulation is a result of the 'delta-t' the temperature differential between the ground temp and the water temp... if you are piping 200* water, you will loose more heat, and have more thawed ground than if you are piping 140* water.
Below is a photo of my insulated pipe system.. two pex-al-pex pipes, inside a 3" corrugated, seamless pipe, wrapped with circular foam insulation tubes, all this inside another seamless 6" diameter corrugated pipe... Seamless is important,, you want to keep any water out of the insulation... water conducts heat very well, air doesn't..that his how insulation works,, it has closed air cells .
So burying the pipe 18-24" works fine,, just insulate it well and seal it from any water.
My water looses 3* F over a 150'run when the water is 180*, and the ground is frozen. Not bad at all.