I see interesting discussion here
The job should be done and the difference in shutdown immediate and shutdown abort is when the job is done.
So
1) Whit shutdown immediate, rollback and checkpoint is done before the database is stopped and that's slow
2) With shutdown abort none of these is done on shutdown, and it's fast. However, on instance startup you need instance recovery (i.e. roll forward and roll back) The rollback however is doen AFTER the instance opens and in parallel so it seems to be faster aproach. I say SEEMS to be since the rollback is still done but with opened instance.
3) If you ussue checkpoint before the shutdown abort, you'll save the instance recovery roll forward phase