First you must understand the differences between other coins and USDT. Unlike any other coins, USDT was not designed to have value based on supply and demand. It was created as replica of USD. So it will be having all the times same value which was fixed at one dollars for one coin. So, it seems most stable coin to you until you understand the idea behind this coin.

Like any other coin, USDT also is subject to any collapse. If bitcoin prices will be increasing then you will be needing more USDT to buy one full bitcoin similarly if bitcoin prices are falling then you will be needing less number of USDT to buy one full bitcoin. Hope you can get my point and you will be using USDT for what purpose it is with us.