The very long frequency types in service are 2D systems only - no height finding capability, and have resolution grids several hundred meters wide. That gives you a very large cuboid of space to examine. In a real shooting war, the size of the systems involved mean they'll be targeted very rapidly and degraded or destroyed as part of the IADS.
They're pieces of the puzzle, not magic fixes.
It would seem like low-freq radar isn't the "magic fix" that the anti-LO crowd (Kopp, Goon, Eric Palmer, etc) describe it as. The L-band radar carried on some Russian fighters is low-freq, but it doesn't seem to have a low enough frequency to be terribly useful -- at 1.5 GHz, RAM is still effective, and half-wave resonance is only capable of illuminating very small parts of the aircraft -- the very tips of the wings/tail (which seem to be the pieces deliberately covered with RAM, further decreasing whatever benefit half-wave resonance gives). So basically, with an L-band system, it seems like the only effect would be that the fighter
might be able to get a
very low-resolution glimpse of the F-22/F-35 (which might end up being rejected as clutter, and isn't a clear enough "picture" to enable a missile launch) at ranges a little bit further than those at which a normal radar would detect a LO platform (i.e. not much further than WVR distances).
As far as low-frequency ground radars, those would appear to be constrained by the radar horizon, giving them limited usefulness against an F-22/F-35 approaching at low altitude. Of course, one could get around this with OTHR, but as you mentioned, it isn't exactly "perfect." In fact, it seems like it has an even lower resolution than you stated -- a OTHR with a 1/2 degree beamwidth (which requires a radar array several kilometers long), when detecting a target at 120 kilometers, only has a resolution of one kilometer! The resolution gets worse as the distance to the target increases, and like you said, they're 2D only.
Edit: I found something rather interesting. Apparently, RAM still works at low-frequencies (just not at VHF frequencies; although the problems with those systems have been discussed):
"When Lockheed started investigating RCS reduction, they developed a RAM coating made of a dielectric plastic with carbonyl-ferrite-covered particles embedded in it. The figured out that the dielectric plastic slowed down the radar waves. While radar waves have wavelengths ranging from around one inch to a few feet while going through the air, they were slowed down in the material. This means the longer waves were squeezed into a wavelength of a few inches, while the shorter ones were a small fraction of an inch long. The thin RAM coating would mostly absorb the shorter waves and would reflect them in such a way that they would cancel out, as its thickness is about one quarter the wavelength of the shorter waves. But what about the longer, more penetrative waves? Under the RAM, engineers placed a thick layer of fiberglass honeycomb, which could be made to have its surface be less dense and its deeper layers becoming progressively denser. This meant that the medium waves would bounce somewhere along the middle of the honeycomb layer, and come out such that they had traveled the extra half-wavelength necessary for cancellation. The longer, more penetrative waves would travel deeper before bouncing, and thus would also travel approximately the extra (longer) half-wavelength. Engineers likened the RAM-on-honeycomb set-up to a multi-channel stereo system: You have a tweeter, which just takes care of the high-frequency (short wavelength) stuff, and you have a woofer, that is best equipped to handle lower frequencies (longer waves). Individually each of them misses a lot of the waves, but together they can handle a wide variety of wave lengths. Typically, longer-range radars use the lower frequencies (longer, more penetrative waves), while smaller radars on fighter planes use medium waves, and missile radars use the higher-frequency, short-wavelength waves. RAM alone cannot protect from longer-range radar waves, so the ingenious graduated-density fiberglass honeycomb layer absorbed some of it and cancelled out what it reflected. It acted as an internal skin, almost always one quarter of a wavelength below the surface. This multi-layered skin is another important feature of stealth aircraft design."
http://www.airplanedesign.info/52-radar-stealth.htm
