Well I wouldn't say there was a massive issue or achilies heel with the BMW engine or Williams Chassis, their success was quite good in such a short time. Teams need a grace period to develop, like Red Bull/Renault have done.
Not forgetting how dominant Ferrari were around the time Williams has BMW, they were hardly bad.
The problems were allegedly down to later politics. Thiessen fell sour when Button-gate didn't pull through. Frank didn't really want Heidfeld. Williams tried to be radical and failed with the walrus nose. Hats off to them for trying to be different. But it all boiled up after a few years of light success and BMW left to pursue their own venture through impatience.
Frank blamed BMW for not having a decent enough engine. Thiessen blamed the Williams chassis.
The early wins and podiums with BMW were promising. The turnaround for the team in the state that it was in the years after they won the constructors championship was a slight downfall, only to pick up again when BMW joined in.
You have to think that if they had stayed and worked through the political issues, what could have been...
They should have had BMW power up until 2009 or so, before they left to buy Sauber out.
It was simply impatience. Ferrari were winning EVERYTHING. BMW wanted results. Immediately.
I guess they realised you can't click your fingers and expect.
Still, Williams Renault is good for countless reasons. They are an engineering team. They will come good again.
Jenson drives it like he owns it; Lewis drives it like he stole it