The biggest criticism I can make about the Avro Lancaster is that it was badly undergunned. I realize that it was designed as a night bomber, but it still could and should have been better armed. First, why were the machine guns .303 instead of .50 caliber? The latter would do a lot more damage to any attacking German fighters (as the Luftwaffe pilots who fought the American daylight formations could attest), yet only a handful of Lancs were so armed, apparently on a trials basis. Secondly, and more importantly, why was there no ventral gun turret? One should have either been installed from the beginning, or at least retrofitted. According to Cajus Bekker's book, "The Luftwaffe War Diaries", at least a quarter of Bomber Command's losses were due to attacks from below. Many of the German night fighters had an installation they called "Schragen Musik"; this involved mounting 20mm cannon to fire upward at about a thirty degree angle. A gunsight on the roof was all that was needed to enable the Luftwaffe pilots to attack the British bombers outside the field of fire for the bomber's tail gunner; in other words, without fear of counterfire.
The Lancaster was a great aircraft, but it was not as good in some ways as it could have, and should have, been.