I haven’t rolled a hunter due to the bug, but my understanding is that you can’t feign/trap instantly, and you can’t feign/trap at all while your pet is still engaged. I only played hunter in Vanilla, and the first week of BC, so I know you could feign trap while your pet is attacking. There’s a work around right now to macro your pet off, but that’ll be a dead give away in PvP…
There used to be a tactic, where you’d jump, hit your feign/trap macro, and the FD animation wouldn’t display because you’re “moving through the air.” It was a good way to feign/trap in PvP without the other player seeing it.
If you don’t let the animation fully complete, then feign death may not have any effect on threat. This can seem very inconsistent because sometimes it will work just fine if you clip the animation. A lot of times when you are rushing to do multiple things, you may end up clipping the animation causing feign death to not work. There may also be any number of things happening to your character that prevents feign death from working. This was fixed in later versions of the game, but not until Wrath iirc.
Also, Feign Death has a chance to fail, just like Vanish and a number of other spells. A lot of hunters may be incorrectly saying that Feign Death is bugged, when it is actually just being resisted by any number of mobs. Resists will happen more when you have threat on a larger number of mobs because each mob has a chance to resist your FD; and if 1 of them resists, the effect will not work for any of them because they are socially linked in their threat toward you.
Feign death takes a significant amount of time to register “Exiting combat”. If you do not exit combat, you cannot use a trap. If anything does anything to you during this massive delay (between 1-3 entire seconds) you stay in combat and the feign death might as well have not happened.
Would also add that if you feign death, a melee swing will still land with a delay. This also keeps you in combat post feign and also prevents the trap. On all 3 of my characters I have not been successfully trapped by a hunter.