I think it got clear in pre-season when they tested this in the simulator. It seems to me that Mercedes really has issues how to deal with the ground effects of the car. I've seen Ferrari also being bouncy (one of Mercedes problems) but are still very quick. Also the back of the Mercedes is not stable they say, which is one of the reasons Lewis in particular lacks the confidence.

He may have jumped ship to Ferrari, but keep in mind that like Verstappen, Leclerc like a loose back and pointy front, which will cause oversteer. So I am not entirely sure what Lewis preference is, but he may have a similar issue when he joins Ferrari.

The lack of confidence is actually quite clear to see, since Russell seems to be more at ease with the way the car behaves than Lewis.