Back in the day, people would usually cave full iron enchants and having a diamond piece would be rare. So if someone is full diamond, they were always the top killer or high up on the leaderboard. Now days, 1.7 games are all "Caves 6.0" and 1.8 vanilla rates are really high. Therefore everyone is usually on surface with full diamond or 3/4 diamond with like 10 gaps each. Personally, this just ruins the idea of UHC. It was supposed to be challenging and difficult. Now the only challenging part is pvp because ore rates and how every game is "Cutclean, Hasteyboys, Timber" at least.
How to fix this?
Not even going to lie, i don't have good idea's on how to fix it but i believe making an ore rate plugin would be great. On the server i used to host on a year ago, the dev make this. You could choose whether to increase or decrease the ore rates and by how much. People used to be full diamond every game, now people are most half diamond after a full game of caving. When I hosted more efficiency scenarios like timber, i would throw in something dumb and punishing like blood diamonds, balance, blood cycle, extreme blood diamonds or genie. (Genie is punishing to the people who cave all game)
Another way to fix this is to add more difficult scenarios. I'm sure (or at least i hope) all of you know the r/UHCHost scenario list. Take a look around that town and find an idea that you like.
Side note: Meetup tp'ing people is generally not good as it gets rid of the natural fights.
TL;DR I don't like how uhc scenarios are easy and games everyone is full diamond fire aspect so make the game harder by decreasing ore rates or add a punishing scenario for every efficiency scenario.
Ore rates are vanilla. They are exactly like in a normal single player game. I am pretty sure the Minecraft diamond ore generation hasn't changed in ages. I won't definitely change the ore generation for all game modes.
What I think that changed is that most of players have better skills, hence more people end up with diamond gear.
If you want the "oldest" version of UHC, play Vanilla UHC.