If you wanted to use radar altitude to keep yourself safe on a non-precision approach (for the layman, non-precision doesn't mean "sloppily flown" but "flown without altitude guidance," what we call "the glideslope," a electronic signal that gives a 3° sloping path towards a point about 1000 feet in from the landing end of the runway) then you would really need a runway with an absolutely flat clearway of considerable length. In this case I believe it has been said that the terrain actually slopes up towards the threshold so that one could imagine first going down to X radar altitude but then being forced to pull up to keep that value. That would be a rather hairy thing to try in a light aircraft moving at about 80 knots. To try it in a large aircraft moving much faster...
About the only recognised way to use radar altitude I am aware of is during a Category II ILS approach, when you set 100 feet radar altitude as your decision height. Part of that procedure is knowing that you may well touch down on the runway during the missed approach if you cannot see it at decision height since you are going to be descending in a stabilised approach on a 3° glideslope at around 500 feet per minute, depending on your groundspeed. So if you are exactly 100 feet high when you make your decision to go around, the aircraft is going to make runway contact in exactly 12 seconds, when there is, of course first the delay inherent in your making that go/no go decision and then the lag in the aircraft's change of flight path.
This is assumed to be relatively safe because you are reading the radar altitude over a flat clearway just before the runway threshold while in a stabilised configuration. (You will cross the threshold itself at exactly 50 feet, normally.) It is nothing like flying towards a runway while trying to hold a steady value on a fluctuating radar altimeter while simultaneously trying to see a runway that is just "somewhere out there" in thick fog. There would be so many variables in that to make it a very difficult trick to pull off safely.
You can see from this that the Category II ILS approach is a rather demanding procedure, and in fact it is usually only allowed to be performed by crews with special training. To try and incorporate just one of its elements, the use of radar altitude, in some sort of improvised approach can definitely get you into serious trouble.
There is an old saying that runs, "The superior pilot uses his superior judgment to stay out of situations requiring his superior ability." Of course that is in an ideal world!