Huh?
Dude, there were 6 known incidences of the F-111 TFR killing pilots between 1971 and 1973. Now, maybe I had access to stuff that you didn't, but this is a well known fact.
Lots of things that are "well known facts", are not true. In any event, who would know better than the guys who had fix the problem, if there was one. Most of those 71 to 73 losses are just "unknown".
Now I know about situation where the TFR might try to fly you into the ground, but those are well known and can be avoided by proper flight planning. (The more modern TFRs will do little better in those situations, it's matter of not getting enough backscatter from the terrain for the radar to detect). None of the losses were in such situations, other than two over water, where the radar altimeter is supposed to control the aircraft when the forward looking radar gets no returns from the water.
Now there might be case or two, although I'm not aware of any, where both TFRs failed. I was speaking of a functional TFR flying the aircraft into the ground. Even if both failed, a fly up command should have been presented and executed. Over most terrain, the system can fail "hard over" and the low altitude warning function, will allow a safe pull up using only 2 to 2 1/2 gees (1 to 1 1/2 "incremental", not counting the 1 gee you get just flying straight and level). An assuming the radar altimeter is working. The LAW won't save you if the failure occurs when you are already in a "full" dive, (actually only about 10 or 12 degrees of dive angle) but that's the reason for all the redundancy, failure detection, etc.
Heavy rain is one possible situation that could cause problems in the early F-111 radars, but only in "rocky mountains" type terrain. Later model radars had logarithmic receivers, those systems would see the rain and try to fly over it, rather than be blanked by it.
I found this declassified 1974 report on the operations in SEA, including an analysis of the potential causes for those losses in theater. In at least the last such case, the crew was recovered, (others were recovered after a stay in the Hanoi Hilton or similar such accomedations) and reported a hit by a "golden BB" which took out the hydraulic system.
In those cases where the TFR was suspect, the conditions were heavy thunderstorms, so it is impossible to know if the radar was blanked by the rain (new versions, but still '70s timeframe, "see" the rain and try to fly over it, due to different receiver characteristics and signal processing techniques) and allowed the crew to fly into a mountain, or if the thunderstorm itself caused the aircraft to go down. Below 1,000' or 500' AGL it doesn't take much of a downdraft to swat you into the strato granite. Two others crashed into the Gulf of Tonkin, implicating the Low Altitude Radar Altimeter, upon which the TF system depends when flying over relatively "smooth" water, and in some extreme situations, not likely to occur when flying out to sea from a relatively flat area such as the coastal areas of North Vietnam. Of the 6 losses, two where definite shoot downs. Four remain somewhat "unknown".
Whenever their is insufficient radar cross section (called sigma0) of terrain, the radar isn't going to be able to see it. The constraints on dwell time, (you have to scan a good sized area fairly quickly, and power aperture (aperture is the size of the antenna, and to a first approximation a bigger antenna can compensate for lower transmitter power, and vice versa.), preclude designing a radar that could see it. (Don't mention sand dunes or snow covered rolling terrain, both are a problem for a radar based sensor). Modern TF systems use digital terrain elevation maps, which allows the radar to be designed for a shorter range, and more sensitivity at those shorter ranges. (You can't depend on the maps entirely, they aren't accurate enough, and you don't know where you are relative to the map(especially in altitude) accurately enough. However the map and GPS/INS sytems can provide you with the early pull information that you need to fly over large features, with the radar, or possibly lidar, to provide the fine positioning information. I've been out of the field for over 10 years now, maybe over 15, what year is it? :) ) So undoubtedly lots has changed. Mainly what's changed is the requirement to do that TF mission in the first place. Now the strike aircraft have all gone stealthy, or are not used until the air defenses have been attitude, at least locally. (Similar to the attrition caused by Army Apaches lead to the Air Defense site by Air Force Pavelow (HH-53 variant). (The Pavelow does have a TF radar, but it's an old one based on the A-7s and RF-4 radars, unless it's been replace by something else when I wasn't looking... it should be, at least by the LANTIRN TF derivative that is used on Army Spec Ops HH-60s and HH-47s).