Both the comments above are different sides of the same issue. If the front is the softest, more roll, then a stiffer rear will simply lift the inside rear wheel and make for instant oversteer. I spun my '77 BMW for that very reason after which I removed the factory rear roll bar and left if on the shelf until the week before I sold the car 20 years later. It looked like a good idea but it didn't do much to improve handling and made high speed handling worse.