When you establish your background and skill set. Specialize. Don't be a jack of all trades and stick to your skills. Don't suddenly pull out a skill just because it's convenient for your character. An Assassin is almost always overpowered this is something on the RPers side as they assume that since they are a Ninja, Assassin or other powerful character in Hollywood mythos. So don't do that.. 

Real Assassins are weak against direct combat. However since they sneak around communicate all of your actions however minor they are to those you plan on interacting with. That way you know what limits you have and can keep others from calling you overpowered.

Once your skill set and background is made, stick with it.