Google's John Mueller said that the Google Search Console API does not do any data sampling. He said it uses the same backend data as you would get in the web user interface. He did add that the API does have limits but added there is no sampling of data returned to you via the API.
Rhea Drysdale asked John about this and John replied on Twitter. Here is the conversation:
@Rhea @googlewmc The API uses the same data back-ends as the UI; there are limits, but there's no sampling involved.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 3, 2017
Most APIs do have limits for spam and resources reasons. Web UIs by nature can control limits nicely but APIs, since computers are asking for the data, sometimes need to be throttled and slowed down, which is where the daily limits come in.
But you should get the same data from the API that you would get from the web interface.
Forum discussion at Twitter.
