Mauricio 'Shogun' Rua is one of the few guys left in the UFC that was a top-tier competitor in Japan's Pride promotion back in the day. He went 12-1 in the promotion (with his only loss coming via injury) and won the 2005 middleweight grand prix at just 23 years old. Since his move to the UFC though, he's had a lot of ups and downs. And in a recent interview with MMA Junkie, he said part of it is the change in rules from one promotion to the other.
Pride allowed stomps and soccer kicks while not allowing elbows to the head or face, and Shogun believes that the Pride rules were actually safer. He also had to change his game up a lot because he relied on a lot of the stuff that's not legal under the current unified rules:
"For sure, the new rules hinder me. think that the rule set from when I fought in Japan was safer than today’s rules. Elbows hurt a lot more than stomps. I had adapted to their rules. So when I came to the UFC, to get comfortable, it took some time. And, truthfully, my fight style relied greatly on stomps and soccer kicks. So to me, the greatest minus when coming to the UFC was the change in rules."
"I hope that one day, the UFC can legalize foot stomps and soccer kicks. It would be great. I miss it. During the PRIDE era, I have no recollection of anyone getting hurt badly from stomps or soccer kicks. Elbows, in truth, really do hurt an opponent."
I do see his point about elbows, but one look at Roger Huerta getting soccer kicked into oblivion in ONE FC might make people think twice about allowing soccer kicks.