I have played a lot of games with Dreamer and Pandora but none with Lynch, so my opinion is only for these 2 Masters.
Mysterious Emissary is a good addition to your collection.
He is a good damage dealer with 2/4/4 damage spread
He can summon extra minion for your army
He can remove conditions from the enemies with his attack
With 6 Df , 10 W and Hard to Kill he is also a good tank
With his Hungry Land Markers, he can block the opponent's movement
And the list can go on .....
If you play Summoner Dreamer, you can use Mysterious Conflux, to boost your minions, If you play with Chompy , the Conflux of Nightmares is must because you can reposition him easily and place him in better and more advance position.
I will tell nothing for Pandora crew, you see the benefits by yourself.
Also, with a little modification, you can use the 2 little girls, on his base, as Changellings.