The main difference between 8th and 9th gen chips is the hardware mitigations (wrt Spectre/Meltdown). So the hardware mitigations may introduce new flaws.
There's a difference though. Cascade Lake (and possibly Whiskey Lake and/or Coffee Lake Refresh R0) have hardware fixes. The original Coffee Lake Refresh has the microcode updates built in, but that could be called hardware mitigations although it's not the same thing.