Number of sites affected
Google does not publish the number of sites affected by the blocking resources warnings in numbers. Instead we have a information that about 18.7% of the total number of websites receive this type of warnings.
Finding blocked resources
The webmaster will receive an alerting email from Google addressing the issues of the blocked resources. This will be confusing for the webmaster because the email does not deliver a clear idea on the blocked resources. For this you can use the fetch tool in Google Search console but it is a tedious process. But you find a sections called Blocked Resources below the Google Index which gives you the blocked resources and the ways to find them.
Many websites do not want their CSS and JS blocked because they don’t want Google to index them. But Google will index both the file types based on their list of supported indexation of these file types.
Third party resources
Google was holding a third party JS i.e. JS hosted on a third party website called from each webpage. The Google Adsense and JS are included in it. At the beginning Google asked the webmaster to use the third party sites to unblock the JS. Also they have provided the robot.txt. The webmaster can find the unblocked resources by the robot.txt without anyone’s help.
Many webmasters do not consider the robot.txt as a helpful one. But Google has provided some additive directives to the robot.txt file in order to allow the Google bot to crawl all the JS and CSS.
These additives allow the Google bot to just crawl the JS and CSS which are mentioned as to be unblocked and blocks the contents which are defined to be blocked in the robot.txt file.
For getting a better SEO ranking, you should unblock all the JS and CSS of your website. This will increase the traffic of your website. You can also make the Google bot to skip both the contents, but it will severely damage your SEO ranking. The last suggestion is to keep these files private in order to sustain your SEO ranking.