Omen version 3 uses the new Blizzard API for measuring threat.
Supposedly more accurate because they have direct knowledge of the mobs' threat tables.
But comes at a price.
You gotta be in combat with a mob in order to view its threat table.
Send pet into a mob, wait for threat to build up, then open fire.
Well, you're not in combat when you send the pet in, so you gotta throw a shot in there if you want to watch the threat build up.
Ok, use a Serpent Sting, or fire a single auto-shot. Now you're in combat, and you can monitor for when your pet has sufficient threat for you to open up.
Don't really like it, but not the end of the world.
Supposedly there's a small lag in blizzard's threat data. They provide hyper accurate data on the threat levels, just not quite as timely as a local addon that's parsing the combat log.
Havent been impacted by this yet, but for anybody who likes to hover at max range and sit at 129% of the tank's threat, you gonna have some issues. Then again, if you're doing that consistently, you've probably already got issues (or maybe a crit rate that's so low that you never burst and grab aggro, lolz).
That's all I got to say about that one.