I question yahoo's engineering chops 4 out of 5 times after interviewing hotshots from search, ads, or research.
After interviewing dozens of folks from yahoo, I've pained the following picture about the engineering there - the vast majority of it is stuck in 1995, some bits of it have evolved isolated from the rest of the tech world - it's steam punk, then there are a few parts that are shiny and new but skin deep. It seems like a sad place.