Haven't used one, honestly. Yet the word itself, radalt, sounds advanced and promising.
Put simply, a radar altimeter measures the vertical distance between an aircraft and whatever ground happens to be below the aircraft at the time. When flying over the sea or a relatively flat terrain, it is a very useful tool. However, what it generally doesn't do is (for example) tell you that there is a fekk-off great big lump of granite just in front of you that is somewhat higher than the height you happen to be right now. Although you can partially get round that problem, it still doesn't solve how you can try to maintain a constant altitude (above MSL for example) using the radalt. Early radalts also had the problem that they only measured in a plane perpendicularly below the aircraft. You can guess what happened when you put bank on or changed attitude significantly.
Just imagine flying over markedly undulating terrain and ATC want you to maintain a specific altitude so they can arrange vertical separation between you and other aircraft in the vicinity. The radar altimeter indications are going to be rushing up and down like the drawers of a lady I once knew at RAF Valley, but even faster (if that were possible). Under these circumstances a basically barometric altimeter is going to indicate pretty much steady state altitude. (Yes I know that's not entirely true, but it's good enough to illustrate this situation and explain why you wouldn't want to use radalt.)