To date, three types of oracle...schelling-point oracle that relies on the owners of a fixed-supply token to vote on the outcome of an event or report the price of an asset...which suffer from slow times to resolution (e.g. Auger); API oracle...centralized entities that respond asynchronously to requests for data or prices (e.g. Provable, Oraclize, and Chainlink); Application-specific oracle service...used by Maker and Compound...relies on a single data provider that the Compound team controls to provide all on-chain price data to the Compound oracle.
Oracles, as they exist today, represent the highest risk to DeFi protocols that rely on them. All on-chain oracles are vulnerable to front-running, and millions of dollars have been lost due to arbitrageurs. Additionally, oracle services, including Chainlink and Maker, have suffered crippling outages with catastrophic downstream effects.