If someone d-throws to up air when you're at low percents, rejoice, as it stales and reduces it's potency drastically. U-air is only a threat if it can kill. D-throw to double b-air is much stronger at low percents(haven't tested the exact numbers but starting at <10%-25% you can d throw to double b-air and single b-air up to mid 60%) b-air also strings very well, against bad players you can hit 4-6 depending on if you can read them. A lot of times you can get in 3 versus good players. D-throw to fair is also strong and the length of time the hitboxes stay out makes it very easy to connect with even if they air dodge. The only time you should really worry about u-air is above 80%, sure it does decent damage however using it to rack up early game damage is a large waste to it's killing potency and really its biggest threat. U-air is so dangerous because of the fact that if un-staled it kills medium weight characters at 95% and the hoo-haa combo forces a reaction and really it's about a 50% chance(depending on if you read your opponents reaction properly) to get a kill at 95%-130%(the kill percent is dependent on the weight, and DI of your opponent)
As for your question about is it dependent on weight and fall speed, yes. Like most things in Smash it is heavily dependent on weight, fall speed, and the DI of the player at higher percents. At 0% you are guaranteed 2 u-airs however as I said above if someone uses it that early, be relieved, they are most likely going to rid you of worry of one of their biggest threats.
