We use Bitbucket server because we prefer the code review UI and the diff views over everything else. I was shocked when I tried the cloud version and saw how different and how much worse it was. Would otherwise actually prefer to move to cloud but not with that UI, will re-eval competitors instead.
I don't know about the other products, but I was always told that Bitbucket is literally 2 separate products with unrelated codebases that Atlassian just branded the same. Take with a grain of salt because I have no good source, but consider that ex. on-prem BB never supported Mercurial (which was cloud BB's big selling point for me), and their UIs were quite different.
I don’t know what the products are like today, but you’re right that they’ve historically been two separate products with totally different implementations.
