Decentralized exchange has a fiat pair too, not only coin/coin. Also the UI/UX isn't that bad, it's still user friendly, people just don't want to learn about it. The first time when they use centralized exchange, they also don't have any idea how to trade their coins and willing to learn to explore the site. So there's no excuse to not use decentralized exchange due to not friendly UI.
Decentralized exchanges are better than centralized exchanges because you connect your wallets to their decentralized exchanges for trading. You own keys of your wallets so in theory, you and your coins are safe.
However, you must know that decentralized exchanges are not 100% safe to use. If you connect your wallet to a scam decentralized exchange and give it access to your wallet, if it is a scam exchange, they can move your coins to their own wallets.
Generally decentralized exchanges are safer than centralized exchanges if you use a trusted DEX.