If you're using minecarts to move yourself around in Minecraft, it's certainly a terrible investment. However, when used as a tool, they are incredibly useful and an excellent investment. Minecarts can do two things that no other setup can: transport items between inventories consistently without the player's involvement, and move entities like villagers around the map. This makes minecarts a really useful tool in the later stages of the game, when you might want to use them to distribute items across an array of furnaces to be smelted or to move villagers to a central location to make trading easier.

TNT is sometimes worth the time it takes to make. Every block in Minecraft has a blast resistance, so if you're using TNT underground, you're running into stone blocks that will shrink your blast radius. On the other hand, TNT can be used on the surface of the desert to get sand quickly and profitably. It can also be used at the lower levels of the nether to blow up a bunch of blocks in the search for netherite scrap.