<As far as engineers go, well let's mention: Mars Observer, Mars Cliamte Orbiter, Mars Polar Lander and the 2 mini probes designed to survive impact on the Martian surface that were never heard from again.

Apples and oranges.

These probes contained many complex systems that all had to operate correctly for the probe to work at all. No one knows what physical condition the Mars Observer, Mars Polar Lander, and the two mini probes are actually in. All we know is that they did not transmit information back. (The Climate Orbiter, on the other hand, burned up.)

An armored reentry capsule just has one job: don't break open under the worst possible final impact with Earth. The technology for this is in hand, similar to that used in nuclear material containment, "black boxes," and burrowing munitions. The only even marginally complex part would be the closure and sealing mechanism, and feedback sensors to detect that the capsule was, in fact, hermetically sealed. Once this is accomplished, the many other failure modes that might occur in the rest of the mission (guidance failure, thruster loss, parachute failure, etc.) won't matter.