Sampling makes most custom reports highly inaccurate
As long as you understand which reports are sampled and which are not, you can quickly figure out which data to trust completely and which to consider an indication only.
For most startups, the 250,000 sampling limit is only a problem when comparing quarterly, yearly etc. If this is the case, just narrow down the cohorts and compare the data in Excel afterwards.
Stick to the default reports. The issues outlined above typically hit harder when you rely on Google Analytics as a part of heavy and automated reports, such as a Data Warehouse, than if you just look at the online Analytics app.
There is always something completely wrong with the install
It’s about realising that insights come in many forms and resisting the urge to base all reporting and performance measurement on the channels that are the easiest to load into a spreadsheet.