Simple physics, a car that produces most of it's aero load from the floor is naturally going to be more sensitive to ride height changes. A heavier car means controlling that ride height becomes a bigger challenge. LMP and GT cars derive most of their aero from the floor, and thus become very pitch and ride height sensitive.
The new cars were designed with the intention of reducing benefit of rake, thus low rake slammed to the ground is the way to go, the same way GT and LMP cars run their aero. It's all about running as low to the ground as the regulations allow you to go. Combined with the mass that's bordering LMP1 territory, and having sampled the car, and how sensitive it is to over braking and over accelerating(things that affect the ride height almost as much as cornering itself, but that's another topic). LMP and GT cars are very pitch and ride height sensitive, however those cars have driver aids that F1 does not. LMP and GT cars would probably be nearly undrivable for silver rated drivers without TC and ABS to compensate for the driver's lack of pitch sensitivity control.
Because there are no driver aids in F1, and the way the cars build downforce, it's going to take some training to get used to the braking and not locking up wheels. This of course says nothing of how the cars will behave when they have to go wheel to wheel with another car.
In my experience, GT and LMP cars still suffer from reduced downforce when following another car, trains often form in real races because of this. The driver aids mask the reduced aero performance, and the larger mass also lowers the reliance on aero to an extent, however the cars are still noticeably affected.
Comparatively, the outgoing legacy cars were so aerodynamically stable that you could afford to slightly overdrive the car, that margin is smaller now. You're punished more for making tiny mistakes than you were, before slight mistakes would cost you a half a tenth on your delta. Now tiny mistakes become two tenths.
This is not to say the cars are terrible to drive, or are bad, they will be almost as fast as the outgoing cars, they just require a different driving style, one that better manipulates the pitch sensitivity of the cars.