0xlmanini
high
The oracle uses priceFeed = AggregatorV3Interface(0x5f4eC3Df9cbd43714FE2740f5E3616155c5b8419) as the source for WBTC's price in USD, although such address corresponds to the ETH/USD feed, as can be seen in the price feed's etherscan page and on Chainlink's feed page.
See above.
The feed for WBTC/USD is set to the ETH/USD feed. Thus, all WBTC collateral in the system is priced as ETH. This affects both the user-facing function USSD.sol#calculateMint() which will return a much smaller amount of USSD to be minted in exchange for WBTC, and it also affects internal accounting functions like USSDRebalancer.sol#BuyUSSDSellCollateral() or USSDRebalancer.sol#SellUSSDBuyCollateral() in which the system must evaluate the value of its collateral.
StableOracleWBTC.sol Notice the addess used is the same as the one used in StableOracleWETH.sol
Manual Review
Given that no WBTC/USD feed is provided on Ethereum, use the combination of Chainlink BTC/USD feed and WBTC/BTC feed