Matomo

Open source, many features, easy to use, and no head aches about data privacy

Self hosted data ensure data privacy which is a good tool. Provides many of the same functionalities that Google Analytics does. Some premium features go even above and beyond. (A/B tests, heat/session mapping).

Works great, I wish the WordPress plugin worked with the mobile app though..

Easy to install, easy to use. Privacy by design allowing accurate reports

Great Analytics tool, but certain things are not free

Piwik (now Matomo) is an excellent opensource analytics engine. Our company runs this on large scale enterprise web software that has millions of pageviews per month. It has offered a relatively stable analytics engine and most importantly provides complete control over the data which is a necessary governance requirement for the the type of work we do.

Some critical concerns: The Piwik API does not conform to modern swagger style open API standards. It can be tricky to understand at first, especially if you have clients who need to use it with 3rd party tools such as PowerBI.

Scaling up beyond a few thousand page view per month is non-trivial. It involves multiple servers and curious complexities such as reading the threads on optimise tables functionality as well as running a separate archiving server and controlling the frequency and type of archiving actions.

Even with the above considerations certain actions can create very high load on a large database, this can lead to timeouts particularly on the front end interface. If for example a user creates multiple custom segments or tries to run a custom date range on the fly. To solve this you might need to get creative with plugins such as custom reports to decrease the scope of individual reports run on the fly and elastic autoscaling events to handle load events.

There is not much granularity on the permissions and roles structures. If you have an enterprise customer who has many individuals who wish to see specific report (but shouldn't see everything). At first you can solve this with specific reports on an email schedule, however an ultimate solution might be to wrap the API in a custom permissions schema controlled by permissions from another app. Overall we would recommend this opensource platform as the best alternative to Google Analytics and the like, just be ready to get your hands dirty in the technical end.