For quite a while now there is this idea of being data-driven and analyzing everything that is happening on your platform in order to identify potential changes that could improve your core metrics such as user engagement.
While this is all great I see one major problem with this style of analytics: the data is not really yours. In a time where user engagement is getting more and more important I find it troubling to store all the data you collected on a service only accessible through a limited API (vs. full featured database access.)
Limited access to this data can slow you down building internal tools like dashboards and custom metrics/reports. There is also a vendor lock-in to a certain degree. While you can switch vendors easily using Segment.io's analytics.js taking your data with you is not as easy as that.
While I'm usually a fan of outsourcing certain complex things (i.e. hosting) I feel like this is one of the rare occasions where people should think twice. Unfortunately I'm not aware of good alternatives. If you are please tweet me.