Hi there. We are currently considering whether to continue using milestones when we move from SFDC to NXT. We’ve had some challenges with the old infrastructure of milestones in SFDC that I hoped would be addressed in NXT now that they live as Timeline activities. Unfortunately, that does not seem to be the case.
Issue no. 1: I cannot choose my identifiers in the load to milestone rule action. This is not made clear in the documentation, but I can confirm that you are limited to Account ID and date as your upsert identifiers. We have a use case where we are loading records that might have multiple identical dates. I need to be able to choose a third identifier in this case. See below for an example.
When I ran this live, only a single milestone was created for the Account ID, and the comment (which I tokenized) only displayed “Comment number 1.” This means that the other two milestones were completely left in the dust because I wasn’t able to select “Summary + Key” as an identifier.

Issue no. 2: I apparently cannot delete or edit milestones created via the Rules Engine? This one I’m not 100% sure on, so I’d love to be corrected if I’m wrong. But from what I can tell, even as a super admin, I cannot delete or edit milestones created via the Rules Engine. This seems like a major issue, not to mention a step backwards from SFDC edition, where I could not only edit milestones one by one from the C360 but could also do so en masse via SFDC workbench. I don’t appear to have either such ability in NXT.
Honestly, it seems preferable at this point to create a custom low volume object that contains our milestones. I’d have control over the upsert identifiers, plus I (and our end users potentially) could edit or delete the data if necessary. I would lose the ability to overlay the milestones on reports, but that’s about it from what I can tell. I hope you consider making these necessary improvements to milestone administration so that I can use the OOB functionality.