Slightly unfair on the Manchester
The Manchester had problems - especially with the Vulture engines- but that is far from the whole story. Avro decided to re-engine the Manchester with the less powerful but more reliable Merlin X engines. As they were less powerful, they extended the wings and added an extra two engines, making the MkIII Manchester.
Why is this important? Well, the MkIII Manchester was renamed the Avro Lancaster. See http://en.wikipedia.org/wiki/Avro_Manchester. Therefore the Manchester led directly to the Lancaster, and there was much commonality between the two airframes.
Indeed, there is more than a little reason to believe that if Rolls Royce had managed to make the Vulture engines work reliably, then we would be talking about the Manchester in the same glowing terms that we now do the Lancaster.
There are much better candidates deserving derision in WWII - for instance the Gloster Gladiator or the Handley Page Hampton.