Just to summarise then, instrument approaches (onshore) are flown by reference to the pressure altimeters only - on QNH or QFE as you wish. Most helicopters have at least two altimeters that allow for cross checking, thus virtually eliminating the possibility of instrument error. Most helicopters have a single radalt (maybe two displays) and thus no means of cross checking a complex electronicky gizmo. An undetectable fault could occur.
However, GIGO rules, and it is possible to miss-set the altimeters. Preparing for an ILS I once had ATC pass me a QNH that was 10mb out. I questioned it (it was unfeasible bearing in mind the QNH at the departure aerodrome and the pressure gradient (or lack of). ATC repeated the incorrect QNH. It was only when I queried a second time pretty much telling them it MUST be wrong, that they apologised and gave the correct QNH. To protect against this sort of error the radalt bug can be set to something below the DH or MDH so that a warning will be issued before the helicopter actually flies into the ground. This is what DB is referring to. It it a good technique for an ILS or CDFA non-precision approach. Yes it can give a false warning on a non-CDFA non-precision approach over high ground (depending on how far below DH you set the bug). But a lack of perfection doesn't mean it is generally a bad idea, and of course these days CDFA should be the norm.