Ok. Two issues to look out for.
1. The Theta 2 litre motor family. There's been a LARGE recall of the entire family of Theta motors. Plugged oil passages blowing the rods outside the block, mainly in #3 cylinder. If you get one, make sure you run the VIN on the recalls.gov website. Many of these motors have been replaced (under warranty) by Hyundai; but only if the car has 100K miles or less. They had issues with the motor, but Hyundai has done right by their customers. A hustle I've been seeing is-dealers will buy up the affected model year Hyundais (wasn't just the sonata, it was everything they produced from 2011-2018 or so) . Get them on their lots, abuse the motors (if there weren't already issues with them) and take them to the local Hyundai dealer. Dealer installs a brand new motor, and dealer gets the car back. Dealer jacks up retail price because of the new motor.
2. The braking system. It's brake by wire and has been known to go out due to bad sensors in the system. Not as prevalent as the motor issues but it happens.
If you're wanting a Sonata Hybrid, go for a 2018 or newer one. Virtually trouble free in my experience.
If you're looking at a specific vehicle, send me the VIN and I'll backtrace it for you. Sometimes I get lucky with a detailed history, sometimes not. If it's been run through our company I can get info on it easily.