As well as my opinion from what I can see on track, it's a stated opinion of Stella.
https://www.planetf1.com/news/mclaren-l ... grand-prix
Detailing the events of the crash after the session, McLaren team boss Andrea Stella explained that it’s clear to see in the data when ‘episodes’ occur when the car is being pushed to its limit.
“But I think in Q3, when Lando tries to squeeze a few more milliseconds out of the car, what we see – and I think we are starting to see these even better in the data in terms of identification of what is going on – the car doesn’t respond as he expects.
“So I think this is a behaviour that kind of surprises him. Today, it surprised him. The car understeered a bit in Turn 4 and he ended up on the outside kerb, and this outside kerb can be quite unforgiving.
“In a way, it’s almost episodical. What’s happening is an episode, and it’s an episode that I think starts from some of the work that we have done on the car – it made the car faster overall, but I think it took something away from Lando in terms of predictability of the car once he pushes the car at the limit.”
Earlier in the season I also though the car was a beast and that the car was the star and that the drivers were not performing as they should, but it feels quite obvious that it is a mix of the two.
There are also statements from Oscar and Lando saying that the gap at Australia was exaggerated because they managed to get laps in that extracted the most from the car. You'll recall that statement made a splash as Lando phrased it as the only two good drivers on the grid, or something similar

. Looking back, I can see the big jump from Q2 to Q3 and I can see that Russell clearly left time on the table. At the time I thought this was deflection, now I realise there's a strong possibility this is the case. There is no evidence that Mclaren left anything on the table though they might have, but it's clear that others did.
They did both make mistakes in the wet at Australia, but the team and both the drivers talk about how much of a struggle this car is. it is evident in the increased number and severity of struggles the drivers are experiencing so far this season. This is clearly part of the cars gift and it's clear that it is a strong candidate for the cause of the issues, especially when running in the wet on dry tyres, when they struggled along with others drivers. The damage and the loss of time were unfortunate.