The problems the previous system had in two games:
Halo 2 you had constant modded matches, glitches, hackers, derankers, and boosters (boosting was probably the least popular way, with how easy modding was at the time, but it was still done). Nobody had a legit 50 in Halo 2. You can ask anyone who actually played back then and they will tell you the same.
Halo 3? Less modding, some glitches, derankers banned and boosting a big issue. People sell and buy 50 accounts and sit on the rank without ever going into the gametype they have a 50 in so they don’t lose it. Some even got carried to that rank. So a lot didn’t deserve it. Some you could say did, but let’s face it. It’s the equivalent of World of Warcraft arena ratings where players sit on a rating to gain the benefits without ever risking anything, and half of them never even deserved the rating.
The system itself, however, is seen as flawless. Why though? A system based on wins/losses rather than your actual effort put forth into the game? What about your actual contributions in the match? Those weren’t factored in. If anything, the Halo Reach arenas had something that made sense. Your effort was actually accounted for in a way. If you got carried, it would reflect upon your rating. So why wouldn’t people have that over 1-50? 1-50 was stupid.