the server isnt really big enough to say its worth adding a game where games take quite long. when looking at UHC in hypixel, its just 10 minutes to prepare for battle, which is already a long time. then there's the pvp time and in worst case even a deathmatch would be needed. its a no from me :c
UHC has his history here on CubeCraft. They added UHC twice and it was a flopped twice. Not because it's boring or the game phases were too long. No it's just that Cube players are not the UHC players type I believe. When they added UHC the second time they added custom terrain in it to give it a more original look. But there was only 1 or 2 custom terrains. Landscape, trees, ores, caves were every time the same. And if you had the right coords to a great cave, then you had full diamond with 10 golden apples every game. Even I was tryhard enough to find 2 OP caves to win basically every game. That ruined the fun for most.
Long story short. I think it isn't a good idea to add it back. There's a high chance that it will flop again and all that work will be, again, all for nothing.