Every post-mortem I read blames an update. I have bought 23 sites in two years and looked at the content calendars behind them.
The pattern is almost always the same: heavy publishing for three months, a slow month, then a two-month gap, then one desperate burst, then the owner calls it dead and blames Google. The traffic chart and the publishing chart are the same chart with a lag.
I am not saying updates do not hurt. I am saying most of the sites I buy cheap were already abandoned.
Currently: 14 sites, 11 of them bad.