I worked as a data scientist for a few big internet companies, and have done a lot of retention analysis (from the ground up). I'd say, it's a hassle for a reason: accuracy. Third party tools seems to give little validation of correct user tracking, and you may have quite different concept of 'a distinct user' and 'retention'. It gets quite deep if you really think about it, both the tracking part and the analysis part. Happy to get into the weeds if you are interested.
Apart from that, I'd also love to hear what hassle free options are out there.
