Beyond this info, I'd add my own suggestions from having spent a good portion of my career digging around in GA...
- If you have multiple domains, sub domains, etc. make sure to spend plenty of time reviewing the cross-domain setup documentation and test it thoroughly.
- If you have high volume, frequently do deep segmentations, use lots of custom dimensions, etc., make sure you have a clear understanding of how sampling in GA works, how to tell if you are being sampled, and find ways to avoid it by pulling reports in different ways. Otherwise you can end up in a situation where you are making decisions off of .3% of your traffic and while Google's sampling algorithm thinks it is fine, comparison against other data sources often shows it is not.
- Make sure any reporting you do across things like GA vs. AdWords is done with a clear understanding of how they each report on paid search. GA reports on it by default on a last non-direct click basis. AdWords just counts everything AdWords touches. This means that AdWords can give you a good sense of where you are gaining traction, whereas GA can help you understand how it works in conjunction with other touch points, and perhaps how you might change the way you weight things and measure success.
- GTM is powerful and free, but with great power comes great responsibility. Also, it can be a real PITA sometimes.
- Annotations are a highly underutilized tool in GA and can save you a lot of headaches. I just wish there was a way to bulk import/export them via spreadsheet or API.
- You can't currently create goal funnels from event-based conversions (please Google add this!), but the workaround for the time being is to push virtual page views at the same as the event fires, and then create funnels off of those.
- User stitching sounds awesome, but is actually much more limited than you'd think from reading overview. You need a separate view (which means your main GA view you use can't segment for the stitched sessions for comparison--just the new view which only contains the stitched users). And there's a 90 day rolling data retention window, so you need some sort of export process if you care about that data. Unfortunately, this is pretty important data if you have lots of cross-device tracking issues.
- Depending on your volume, you can reach the hit limits of the free tier pretty quickly if you start tracking a ton of events (since they all count as hits). Here's a good overview [1] of what these limits are, how they work, and what they mean for you. When I got the scary notification, Google was exceptionally unhelpful in working with me to resolve the problem, despite considerable ad spend. After reducing them to what we thought would be fine, they were unable to assure me that our data would not be nuked, and basically couldn't give me any real info beyond "this is the policy." Super frustrating.
- If you have good logging of events that tracks both server and client-side, it is healthy to compare for variances monthly or quarterly. You'd expect client-side tracking to break more often than server-side, but it is important to see how much that can alter your numbers.
[1] https://www.e-nor.com/blog/general/hit-count-in-google-analy...