Let's say you are a tank and you have 300 armor, 100 base and 200 bonus. 100 of that is going to be ignored by the 50% armor pen and then the LW will ignore 70 more plus any other pen you may have. That leaves you with 130 armor, just 30 above your base. In other words you are now basically Fiddlesticks.
If you are a carry, you MIGHT have 125 total armor, 75 base and 50 bonus. So total of 60 reduced to 65. You are below base armor now and with his insane crit passive, Yas now basically 2 shots you in 1 second.
It's not a hard concept to grasp.
As for the landing, I think that is purely animation and not game impacting. I tried looking it up but there isn't any info I could find saying one way or the other.