I consider them safe for the most part but there is always risk associated with not having control of your private keys. Look at each individual centralized lender and see who they custody their assets with. Coinbase Custody, BitGo, Fireblocks and Gemini are all incredibly secure. BlockFi uses Gemini, Celsius uses BitGo and Fireblocks, and Nexo uses BitGo. The high interest you receive in stablecoins is largely due to high demand for higher interest borrowing due to crypto volatility (ie borrowing for trading etc). Eventually when major traditional banks allow centralized lending/borrowing with crypto collateral these rates will decrease. High bitcoin interest was generated through various means by various lenders. BlockFi for instance was using the GBTC premium to generate interest for depositors (although they had to massively decrease their interest rates due to the price of GBTC now at a discount to NAV). Celsius is also heavily invested in Bitcoin mining which is one of the reasons they can give out the highest BTC yield.