More and more, for companies that have a minimum size, I tend to foster the use of separate tools to do what they do best. Meaning using a web analytics tool to capture the source of the traffic and the sessions, sharing common ID's between Marketo and the analytics and reconciling both in a datawarehouse. At the end of the day, it be less complex, and provide better, more comprehensive results, easier to leverage since the datawarehouse can easily be queried with a BI froint end (Google Data studio, cliq, tableau, ... You name it) that provide a top notch reporting service.
Emphasize needs to be placed on tagging correctly the inbound links (the UTM's) and share enough data between the analytics and Marketo to make the reconciliation accurate down to the interaction level. This means sharing form ID's to Analytics via GTM form instance.
-Greg
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