One issue I usually see with these services is related to not being able to send out email campaigns with "dynamic content" pulled from the client's DB or APIs (based on specific user/list segmentations)

Had cases where the exact same campaign or template had to be created multiple times due to the client wanting a slightly different intro/closing or even entirely different products and services promoted based on user/account preferences.

Sure, it could be managed with multiple campaigns/templates/lists but these duplications could easily be avoided by using slightly more advanced "segmentation logic".

This is usually the case where tracking/stats aren't that important but rather making sure the user receives relevant content.