I still didn't have a chance to learn what happens when I fail to meet the task. A fail was close once though, and it gave me an additional thrill, actually very much of it, and I liked it.
The story: I sank 4 armed merchants (Blockade of Germany), but needed 1 more to complete the task of sinking 5. I started the patrol on February 1st, deadline was March 1st - I had 1 month, so this didn't seem to be a close deadline... well... until I realized the waters North of Ireland were empty. Nothing, null, zero - 3 weeks of search and I got just a lousy coastal tanker, which almost got me killed in shallow shelve waters. On February 21st, with almost half fuel gone, far from home, with no success, with deadline in a week and Uncle Karl waiting for his damn report, with zero visibility, rain and winds of 15 knots I felt very, very desperate. I don't remember such moment from SH3 (although I remember many other great moments).
If I had such patrol in SH3 I'd just feel slightly more bored every next empty day. Here, in SH5, it was opposite, despite lack of any action. Go figure.
[this is not a rant against SH3, I LOVE it]
Now I expect even more thrills, as more hard tasks are coming - Operation Weseruebung already started.
EDIT: The story lacks the finale - I found and sank the 5th one on February 22nd (23rd?). Felt good.