The engines were fine for the most part (sans 2005 V8) under Ford ownership which is expected by Volvo, where they fell down were the transmissions. Volvo went up market (which it need to do) with the S80 and XC90 T6 and shot itself in the foot. They lost tons of repeat buyers and potential new buyers due to all the bad will. Volvo should have replaced every one these transmissions (no charge to the owners) until the cars reached 200K in my opinion as you can NOT put a price tag on good will.
Then even dumber in my opinion was to give up on RWD. Look at ALL the sales between the Lexus IS, Catera CTS, BMW 3/5 series, Infiniti G series - a market position could have easily stepped into if they kept funding a RWD platform.