Retroactive rainwater collection was broken around 0.C, meaning you had to stand near the funnel (less than about 60 tiles from it) for it to work.
Drizzle fills funnels at half the speed of rain. Storms are the same speed as rain.
Proper funnels (not makeshift, not leather) are 20 times better than makeshift and leather ones. Yes, 20 times - not a typo here.