I've been using this mod for a while and it's an amazing mod!
However, I think that the loot that you can get in the mineshaft is a bit too good.
Here's what I think should be changed to make this structure more balanced:
Loot from Gold Chests:
1. I think you shouldn't be able to get up to 6 Life Crystals from a single chest. I can usually find the structure very early in the game and getting 6 Life Crystals is too good for that stage of the game. I'd include only one Life Crystal in a chest. That's a lot more balanced. Maybe you should also be able to find a Heart Lantern in one of the chests.
2. In the 1.4.4 update the Ammo Box was retiered to be obtainable in hardmode. I think it should be removed from gold chests as a result when the mod is ported to 1.4.4. A Sharpening Stations is still obtainable in pre-hardmode.
3. It'd be amazing if you could find an Enchated Boomerang in the mineshaft. It doesn't generate in gold chests anymore and is craftable from a Wooden Boomerang and a Fallen Star (one of the changes introduced in the 1.4 update). Unfortunately, a Wooden Boomerang can be hard to find (even though you can also buy it from the Skeleton Merchant at a certain phase of the moon), so if you are unlucky, getting a Flamarang or a Trimarang can be very annoying.
4. Maybe you should also have a chance to find a Geode or two there as the secondary loot.
5. Putting some Bombs there as the secondary loot would also make sense.
Loot from Shadow Chests:
1. In the 1.4 Arkhalis becane a hardmode item that has a chance drop along with a developer vanity set. Terragrim was introduced as a pre-hardmode alternative. I think you should be able to get a Terragrim in Shadow Chests, not an Arkhalis.
2. I don't think you should be able to get the Ankh Charm materials in these chests. Finding them can be a bit cheaty. For example, the Deerclops boss inflicts a Slow debuff on you, so getting a Fast Clock in pre-hardmode can make the fight a lot easier. The Ankh Charm materials that are obtainable in pre-hardmode (a Bezoar, a Nazar, a Megaphone and an Adhesive Bandage (the last two can be obtained using the Shimmer)) are fine though.
3. I think getting a Dual Hook, a Beam Sword and a Lucky Coin in pre-hardmode is overpowered. These are hardmode items and they should stay hardmode (the Beam Sword even got buffed in 1.4.4).
4. Not sure how I feel about finding all three angler fishing accessories or all three angler informational accessories in one chest. I think you should only find one of them in one chest. You can convert it to the one accessory that you need using the Shimmer, so finding even one is not a good thing.
Also, I have a couple of suggestions for items that you should get in the Floating Altar Chest (when the mod's ported to 1.4.4):
1. A Celestial Magnet is now a item you get from the Skyware Chests.
2. Maybe you should get some Clouds too.
3. Maybe the should be a high chance to find Fledgling Wings there. They were too overpowered to be found in the early game, but I think they are alright as a post-skeletron accessory.
I don't want to spoil what you can find in the structures to the people who may check this thread. That's why the text is hidden unless you click on it.
I hope you'll find my feedback helpful. Good luck!