Hello!
For what version of minecraft are you talking about? Java or Bedrock?
Since you mentioned "1.8 pvp" I guess you are talking about Java.
When it comes to UHC, I dont think a really long taking game like UHC quite fits the gametypes cubecraft already has. Cubecraft has a lot of pvp games, but almost all of those games are quick minigames which take less than half an hour, if no-one is camping! However, UHC you forces you to play for a longer time in one game, and I dont think a lot of players in the already relatively small Java community will play it a lot!
And when it comes to SG, I think it works fine as a featured game. If they would release it permanently, chances are the playerbase will quickly die down (which will also be the case with UHC im afraid). By making it a featured game, it can only be played once in a while, which preserves a lot of the fun imo.
But if I had to choose 1 over the other I would go for SG, since that game is still quick and its not that much work to implement it I guess
But these are all a bit based on my opinions and how I see the Java community
Maybe on bedrock it will work, since that playerbase is WAYYY bigger!