Most TiVos (all?) are contract manufactured. My Series 2 was made by Samsung, and the cheap fan predictably failed after 2 or 3 years. Not long thereafter, the unit became flaky due to overheating; on a hot day it shut down. I disassembled first the unit and then the fan, and lubricated the fan. Reassembled, it worked fairly well, but the overheating had caused permanent damage and the unit completely stopped working a few months later.
My point is that TiVo isn't exercising sufficient control over their contract manufacturers/licensees, and the bad hardware, not TiVo's design, is responsible for poor reliability.