Jester assassins could work, if done right. Jesters have access to the royal court, after all. Typically the role requires them to be both the wit and the punching bag of the court. But the only way it works is if there's just one or two killer jesters. Once it comes out that a jester, Giacomo, has openly assassinated someone of rank, all of the jesters become immediate suspect. 

I'm sure a few kings would kill theirs on the spot, just to be safe (and [i]they[/i] would be the lucky ones, as they wouldn't face the torturers). Should the jester Giacomo ever dare to show his face again, he's sure to be cut down.

Really, the issue with assassins is they have to hide, because there's no way they can face an army on equal terms. They're like the passive-aggressive of fighters, striking from the shadows, using poison, seeding bad intelligence, setting traps. Since they won't openly face a warrior, they're generally despised.

A race of assassins can't hide.

A race of warriors, on the other hand, doesn't need to. Because they fight openly, they can be respected - even by their enemies.

To be a lone assassin or even a clan of assassins, it's possible to hide among the others - as long as they don't do anything to expose themselves. Generally, it means working through intermediaries, poisons, disguises (not just for themselves, but for their weapons and the tools of their trade as well). So if someone drops off a letter for the unexposed Giacomo with a certain tavern owner, it's expected to reach the hand of the assassin within a day, if not the hour. A couple of meetings later, terms are discussed, partial payment made, and Giacomo is on the road with no one the wiser as to what he's about to do.