Ec 225 auto-pilot
On this aircraft, doesn't really matter wether you're flying with ALT (baro-altitude) or Cruise height mode (using radalt).
If you keep coupling modes on, there is no way for you to go into terrain, as the aircraft wil not let you go. Even with only V/S down to the ground, the A/C will level-off at 80 ft.
Of course, if you decide to fly manually, you remove all the safety devices.
The reason why ALT mode is used rather than CRHT, is that the automatic go-around function is very different if your vertical reference is baralt or radalt.
With ALT mode, you go for a "Go Around", and climb to the moon.
With CRHT mode, you go for a "Trans Up", and climb to 300ft, unless you've specified something else.
Hope this helps