Yup, DDs are dumb.
Like you said, DDs are completely ignoring you when half of the convois ships are burning.
I made a mission in the editor where a Fletcher DD with the highest difficulty setting "Elite" and late war equipment is approaching my sub which is at periscope depth.
I went ahead flank and fully extended both periscopes. He did only notice me when I was crossing his bow not more than 200m away.
It was so rediculously easy to shake him off again. I dove to 50m and kept going at half speed - no silent running enabled. I heard him only ping me once. Within two minutes he lost contact.
On the second test run I passed him very closely with ahead flank -> he didn't notice me. When I shot a torpedo at him (that only slightly missed), he finally turned and searched for me. I managed to sink him pretty easiliy while he was circling cluelessly around my position. Some depth charges (the ones that he can shoot from both sides) were pretty close, but it looked like he didn't know my exact position because most of them were "aimed" only in my general direction (I always stayed at periscope depth!).
