Hold on. Let me parse this.
"Level 19 Scoundrel does not get focused when out of stealth."
Really? Because I mean I know I focus level 19s ALL THE TIME in PvP.
When you get high enough to have developed a reputation (40s at least) you'll find the other team -- particularly good players -- will expend a lot of effort trying to keep you out of hiding if not focusing on you the second you drop out of stealth.
The real skill in Scoundrel/Op play isn't button-mashing, it's running away. The better you are at pillar humping and fleeing the more successful you'll be. But yeah, expect to get focused if you're any good and you keep PvPing. They want you out of the fight and fast.