Newey actually said the RB6 was the car with the most downforce in the history of F1.
It actually finished in 33/38 races it competed in. 3 of those DNFs were due to accidents though.
The RB5, RB6, RB7, RB8, RB9 cars were all fantastic at times, but not all the time. The STR3 was not as good as those, but had its moments in the sun in Sebastian's hands. But it is not the cars that will get the scrutiny from teams - they are not hiring the cars - they are hiring the driver.
It is how the driver drove those cars, whether they were good, bad or otherwise. Did he extract the most he could from them? The RB8 and now the RB10 were initially a handful for Seb, his RB6 initially unreliable, but how did he handle that adversity both on track, in the garage and in the press?
That is how you look at hires. If you wanted to hire a regular house painter for your ongoing business and you looked at his work, noting that he'd had the use of the most up to date and fantastic tools, which your company could not afford - would you shrug him off figuring he could only handle that particular variety? No, especially if you knew he'd started out with less fantastic tools and still did stellar work (i.e., STRF days). You look at the quality of work - was it a slop job? inconsistent? misapplying the colors? using the wrong techniques? and the quality of business, poor customer or employer relations? acumen for the business, etc.
So it is the same for prospective drivers. Sebastian is not perfect, but he clearly has a lot of "yesses" that can be checked off in the positive attributes category. This is true of other drivers as well, but the idea here is, who in this category of drivers is available at the time of need. I disagree every team will want every top driver anytime - there is a right time and situation for all. So we have to wait and see what the future holds, but one hears noises, and rumors are never far behind...