I use other counters besides flagcounter simultaneously on my website (statcounter and google analytics).  The numbers mostly match up within about 5 to 10%.  I have no idea which is more accurate, but I don't see any pattern that would suggest that flagcounter is always undercounting or overcounting visitors compared to the other counters.

1. General: Freedom from error (correctness), or closeness to truth or fact, resulting from exercise of painstaking care or due diligence. Accuracy depends on how the data is collected, and is usually judged by comparing several measurements from the same or different sources.

2. Accounting: (1) A financial statement item is judged accurate when all account balances included in it are correct in (a) value, (b) presentation, and (c) disclosure of material information. (2) An account balance is judged accurate when all elements included in it (such as assets, costs, equities, liabilities, reserves) are correct in (a) value, and (b) classification. (3) A class of transactions is judged accurate when all accounting events included in it are correct in (a) value, and (b) description.

3. Engineering: Ratio of an error to the range of possible output (full scale output) values.

4. Forecasting: Degree of fit (matching) between the predictions and the actual data.

5. Testing: Degree of the closeness (to actual value) by which an instrument measures or senses the value of a variable being measured or sensed. See also precision.

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I have been using flagcounter for more than 1 1/2 years, and so far it is precise and accurate.
In my opinion, the only thing that might look 'exaggerated' is the pageviews since it counts each click. But now since there are new feature of 'total unique visitors', that is not a big problem then.