Another issue that comes up a lot, is when the browser the site visitor is utilizing has an ad blocker. This can allow the data to be inaccurate and not allow for proper tracking.
The way to overcome this is allowing your analyitcs to be hosted locally. The reason this is an issue is because a lot of people will host them on a cloud based system.
When you set up the data analytics filters, there are multiple filters you can enable or disable. One issue is when you have the incorrect filters enabled and this allows for inproper results.
To further correct this issue, run tests to ensure the data is accurate. For example, there is a filter that may not allow tracking from company IP address. If you can run tests than it could save you in the future.
I believe the biggest issue with data analytics, is that there are not as many people that actually understand how to set them up and report the correct results. This is obviously something that can be overcome, but until it is we will always have issues with reporting the correct data results.