This is a very broad question and some aspects will be subjective, so it's difficult to give a complete answer. There are a couple of obvious problems with making charts and graphs accessible, notably the reliance on colour to to provide information and the fact that the data is represented visually.
Google has mitigated the colour coding issue somewhat by including tooltips, which appear when hovering on elements of a chart. In the example below, while it might be difficult for someone with colour blindness to differentiate and cross-reference the segments against the key, they can still hover on each segment to reveal the label and associated numbers.
The visual representation of data is still an issue for those using a screen reader or braille display. Fortunately, as the original question mentions, Google does output the same data in tabular format. This is hidden visually and identified by an aria-label element, which is announced by assistive technology:
aria-label="A tabular representation of the data in the chart."
I haven't tested Google Charts in any great detail, certainly not all of its methods and features, but I expect most accessibility issues have been mitigated directly or by providing an alternative WCAG conforming version of the same content.
