I run a small exchange in my fandom, and in five years and probably about 20 forced defaults at the deadline, I have only ever had two people turn in a fic a few hours/a day late because of being mistaken about the deadline. The other 90% of people I forcibly defaulted either forgot about the exchange, decided they didn't care about the exchange, forgot to default on their own or didn't know they could. The YT percentage of people who do this may be somewhat higher because of the no-default deadline + the posting deadline being at a weird hour of the morning for people in the US, but I would be shocked if much more than 15% or so of forced defaults were people who have a finished/nearly finished fic to turn in.