RedBull is stable.
The problem with it it is tricky drive fast!
Gathering up comments from Perez, Max and other engineers it seems:
The RedBull aero platform has a high operating downforce in the corners, but you have to keep the aero platform stable to unlock it.
Max is very good at feeling the gyration of the chassis throught the corners so he knows when he can be aggressive in braking and direction change and most importantly, when he be gentle in braking early and slowly feeding in the steering to keep that big Newey raked floor right where Newey wants it. Max in an incredible driver in this way he feels the car and learns the car.
Perez is one of the best brakers, just like Daniel. Perez is also very good on throttle application so he suits the point and squirt rear limited tracks. The other thing Perez is good at is feeling the mechanical grip in all four corners. All this culminates in super ability at saving tyres. Downside for Perez and Daniel is that they not good at feeling the aero platform behaviour of the car.
I think a driver like Bottas, Leclerc or George would get on top of the RedBull in qualifying because those drivers are known to drive "small operating window" cars too.
So yeah. Max has better feel of the aero platform and knows to balance aggression that's my take on him over the years. I don't see the RedBull as unstable per se, (it wont drive you off the road like a Haas or Williams!) but difficult to unlock the upper operating window.