@captainwilks
The gold hoarders, merchant alliance, and order of souls will offer you an emissary quest once you reach grade 5 with their respective emissary flags, however you have to acquire the voyage from the correct representative at an outpost after reaching grade 5, and you have to keep your flag to keep the voyage. If you lower your flag or sink and lose your flag, you'll lose the emissary voyage.
Reaper grade 5 emissaries gain the ability to see all other emissaries on their ship's map table, instead of receiving a voyage.
Grade 5 guild emissaries don't receive any bonus by reaching grade 5, but they do appear on everyone's map once they hit grade 5. The multiplier for guild emissaries is also maxed at +75% instead of +150%.
I will likely never do a merchant emissary quest. They are not worth it at all.
The order of souls emissary quests are ok but you are unable to get other voyages to appear at the same islands as your emissary quests, so if you have an order of souls emissary voyage at an island, it will likely only be that voyage at that location.
Good hoarder emissary voyages can actually double up with normal gold hoarder voyages. If you vote on a gold hoarder voyage right before sailing to an island that has your gold hoarder emissary voyage on it, you can increase the amount of chests avaliable on that island. These are usually the only way I do emissary voyages now.