I can't speak to the Rugby stuff...
but most of those football plays were great athleticism, bringing down a giant of a man headed the other direction isn't easy. However, many of them are illegal under current rules (you can't leave your feet to spear a guy, helmet to helmet contact is illegal, can't grab a facemask, leading with your helmet is illegal).
A cover corner is probably the toughest job in sports.. you have to keep up with the fastest guy on the team, the entire scheme of a passing play is based on confusing the shit out of you, you have try and read a QBs eyes but not be fooled by the QB trying to look you off the ball.. and if you screw up and your guy gets open you have to have the ability to correct while the ball is in the air and get back to where you are supposed to have been in the first place to break up the pass... and the best ones are so good no one even bothers to even test them by throwing their way.
I wish I could find a good video of Asante Samuel at work, but they are all just interception highlight reels.. which are impressive, but his ability to turn change direction and take off running is just amazing.