Terra UST was a stablecoin and it was safe until one day it suddenly wasn't. Something similar can maybe happen to DAI tomorrow or Tether. Centralized or decentralized, they might still get de-pegged.

Anchor Protocol was a big factor in the UST fiasco. Waaaay too much money was locked in the platform where interest rates of 20% were being offered. Unsustainable interest rates that started dropping after a while. When that happened, people lost interest in UST. That tells you that for 80% of the investors (that was reported as the percentage of the total allocation locked in Anchor Protocol), USTs only use case was to earn those 20% extra rewards. And that's it. Take away or lower their rewards and the system falls apart.