>>33586492The underground was extraordinarily bland looking, but I think it's commendable that Gamefreak wanted to give you a wide open area to explore, build a house in, play a mild PvP game within, grind for mats in a fun way, and bury spheres that would grow in real time that was essentially like poke-pelago and the beans.
If they made the actual underground map more varied and interesting and perhaps expanded the capture the flag mini game it could be a great way to increase the post-game fun.