Hidden costs nobody mentions for under-construction projects

Okay I'll be honest — buying property is way more confusing than I thought. I'm looking at Greater Noida, specifically Adarsh Group Hindon Vihar and Eldeco Live By The Greens Phase III. Adarsh seems like a good ready-to-move option, but Eldeco is ongoing. My parents keep saying under-construction mein hidden costs bahut hote hain. What's the real total cost of ownership for Eldeco, beyond just the flat price? And for Adarsh, are there any unexpected maintenance charges ya registry issues? Ab samajh nahi aa raha which one is better long term.