Please watch my short video illustrating the experience.   
https://www.youtube.com/watch?v=wPUAkG78BFE&feature=youtu.be

Scenario 1:

On MacOS X Sierra when setting up SourceTree for first time and choosing "SSH" as the authentication method, SourceTree:

1.  Should not have a URL for the Bitbucket server, should be a hostname without the https://
2. Should not need a "password"
3. Generate SSH Key button is greyed out
4. User can skip setup and perform this step manually through the Remote Repository Browser's "Add Account" function

Scenario 2

On MacOS X Sierra when using the "Add Account" function the experience when selecting SSH as the authentication method is:

1. URL is asked for, should be a hostname
2. Password is not needed
3. User should be able to select an existing ssh key on their system
4. Generate Key button is visible and clickable, when clicked, process continues, but no key is generated and a "Tooltip" is displayed when account is saved indicating no SSH key
5. When editing the account, and selecting "Generate Key" it now works (but generates a public/private key with a "space" in the filename (e.g. mwilde_Bitbucket Server.pub)

After putting my public key in my account on Bitbucket and trying to clone, errors are related to what is in the ".ssh/config" file this occurs.

1. No hostname is in the ".ssh/config" file (edited by SourceTree) causing a failure on clone 
2. When hostname is added, errors occur due to "space" in filename
3. When ".ssh/config" is edited removing spaces and subsequently renaming the private key to match the reference in the ".ssh/config" file

After "fixing" all of these issues, cloning only works thru the CLI, or via a "Clone in SourceTree" link up in Bitbucket Server.   Cloning does not work through the remote repository browser.

Please watch my short video illustrating the experience.
https://www.youtube.com/watch?v=wPUAkG78BFE&feature=youtu.be   

Note:  I have a few hundred people in my company i'd like to train on SourceTree (and have them use SSH keys if possible), so please fix this ASAP.