I just want to provide a comparison of HP execute thresholds. Elder dragon executes at 20%, Urgot ult executes at 25%. Just watch the first clip and try to guess when the execute happens.
Surprisingly low, right? And elder is 5% lower. In addition, the execution in the video happens almost immediately at 25% because Urgot's W is dealing many ticks of small amounts of damage. When large amounts of damage are flying around in the late game, champs won't always be executed by elder at exactly 20%, it will often occur at even lower percentages after their health drops from maybe 21% to 17%.
As an Urgot player, I know the feeling of mashing R to reel them in, and sometimes I can't believe they aren't low enough. Thought this might give insight to the strength of Elder.