The Speed 12 took awhile to tame in GT2 for sure. I used to just stretch the gears out on it, that worked very well. It had the same snap oversteer issue Cerbera LM had, only exacerbated by the extra power.
In GT6 it responded well to rear ballast and SR changes. Like many cars it has been PP nerfed sadly.
I say the overall worst cars are the NSX-R, GSXR/4, and Chappie 2J, with the 2X and Hybride/LM55 being runner ups.
Oh they drive great every one, but you just brought a main battle tank to a fist fight essentially.