Chugalug 2
"Unfit for purpose"
Well, let's start with the Avro Anson as an anti submarine aircraft. The old ATC song went (IIRC) "The Ansons an aircraft that is much maligned - it hunted U-Boats in the last War you will find - but if the sub put on speed it left the Anson behind: its the pride of the ATC". In one memorable incident in June 1940 3 Ansons not only survived an attack by 9 Me 109's but succeeded in shooting down 2 and damaging another. It was an Anson of 500 Sqn that made the first attack of the war on an enemy U-Boat.
Gloster Gladiators were off loaded from FAA Carriers in the Med to make way for Fulmars (basically a Battle with forward firing machine guns) because the new doctrine was that only monoplanes stood a chance in modern warfare: but "Faith" "Hope" & "Charity" played a gallant role in the defence of Malta.
The Curtiss "Tomahawk" and "Kittyhawk" gave vital support in the Western Desert although vulnerable to enemy fighters and were not finally superseded until 1944.
It is not always the aircraft that is at fault - but the mindset of those whose duty it is to use those aircraft in the best possible way.