I'm not sure why people complain about digging for core fragments. You should be digging anyway; it's the only way you're going to find enough ore to get you started. You can find weapons and vanity items. A few times I've found oil, allowing for an early lamp on a stick.
I typically enter the Erchirus Mine with every base tech unlocked and at least a couple of found weapons, thanks to digging.
the complain comes in the form that in order to get spaceborne you have to complete a quest that doesnt even give you the materials completely. no, you are obligued to reach to the planet's core in order to compensante RNGisus bullshit.
now. i wouldnt mind going the extra mile to unlock the entrance to the outpost. but seriously, the underground is boring. the ore volume in the clusters is mediocre at best. and there's not much to get your interest in digging further deep.
do you see larger amounts of metals? not unless you reach a tier 4 system at minimum
do you see unique items? rarely, the only legendary weapon i've got in my playthrought was the alien worm gun that outside of the uniqueness its completelly useless
do you see interesting landscape? barely, specially in the starting planet. guaranted to have a layer of desert and clay. but nothing more. i've seen what seems to be jungle and found a large tar biome once in a toxic planet.
the only interesting stuff on them are the blocks. no special features like underground cabins or minidungeons. that only happens on the surface biomes.
again. i dray my complains because of Terraria. you can dig and dig and dig on a terrarian world and you're guaranted to find either large clusters of ore, a unique mob with interesting drops, a minibiome with plenty of stuff to loot or a cabin with a golden chest.
sometimes you find a mushroom cavern that gives some use in the form of making second tier potions out of the mushrooms. and its unique mobs.
the mob diversity in Starbound is horrible. most of the minibiomes only have 1 unique spawn. the rest of what appears above and under are randomly generated things that dont seem to have a feature related to their respective biomes outside of color.
and all of them drop the same stuff. is it an insectoid? then hardened carapace, is it half plant? living root, i can understand the homogenity of drops due to being the same type of monsters but at least the unique mobs should drop something that makes the biome worth looking for.
most of the time you find just a chest sitting there with randomize loot that its either cosmetic and useless, or its furniture that its also useless unless you want an specific colonists, which i doubt will be of help to your mission.
so yeah, there's plenty of stuff that should be fixed or expanded regarding the underground.
the game may entice the player to explore other worlds. but whats the point of it if every planet has the same boring stuff under its crust? you enjoy more travelling on the surface and going all the way around finding crops and settlements than trying to find stuff underground.
the only reason i go to the depths is for metals, because buying them from the Ursa shop is expensive and pixels are a pain to obtain if you dont do missions or loot everything on sight.