Old combo system was significantly worse. It was a variable speed tempo game with modifier buttons as well. Your mods and other player's abilities would mess up the tempo needed for the hold and the delay based combos, or cause the delays to be so short you couldn't execute other combos which are modifier based.
Only a 6.25% chance for a 4 man rad share, not too bad really. Unless you mean they all got the exact same reward, then it is a .077% chance or around 1 in 1300. Quite rare, but not unheard of with how many chances there are all the time.