'Shutdown abort' is a very inelegant way to kill the database. I think you should use a 'shutdown abort' only when there is not another way to shutdown the database. For instance you have applied a shutdown immediate and the server doesn't response after a lot of time.
Remember that after doing a normal startup and shutdown will allow you to know if the database is going to operate correctly after a 'shutdown abort'. Shutdown abort will crash your database.

* If you use a normal shutdown, the database will not go down until all connected users have finished all their work and also they have been disconnected by themselves from the database.

* You can use a 'shutdown immediate' if you don't wan't to wait the users finish their jobs. I use this option in my night's coldbackups because I know the users are not working in that moment so it's a good option.
If you use a shutdown immediate, the current transactions of the users will be automatically rolled back and after that the users will be disconnected automatically from the database.

* If you use a shutdown transactional the database will not go down until all connected users have finished all their current work (current transactions). After that they will be disconnected automatically from the database.

Regards