Physics are terrible

You need to provide more context behind your stability issues, as making a blanket statement like that is not useful at all to the devs.

Can you show a clip with an example? Provide the setup you're using along with your controller input settings? Is it happening during all corners on all tracks, or just specific ones? High, medium, low speed corners? Straight line or trail braking? What are your tyre pressures? Are you overheating them?

I see so many clips on YouTube of people complaining about snaps of oversteer under heavy braking when they're using 51% brake bias for example. You need to up that to 54-56%. Suspension stiffness and Roll-Bars have a big impact this year on how stable your car is in the braking zones, along with responsiveness and stability mid corner. 

Although not perfect, this is one of the better handling models we've had in recent years and I feel like the majority of complainers are not utilising the setups properly. Time Trial setups are AWFUL in a race, and yet most players are just copying whatever is currently sat top of the leaderboard, rather than tailor a setup to suit your own driving style or race needs. The TT World Record Setups are there purely to get that car around a track as fast as possible over 1 lap and will exploit every setup parameter possible to do that, They're also extremely unstable and require a great deal of skill to control the car. There are drivers in my league who clearly just use TT setups, qualify well and fall apart in races with no pace and constant handling mistakes.