Maybe Assassinate is the problem, rather than sneak attack? I can imagine that if you allowed this then an Assassin/Monk could do staggering amounts of damage in the first round of a combat, every single combat, all day, with no short-rest or long-rest restriction.
Possibly, but the Assassinate feature already synergizes with flurry of blows. Sure, it would be better if you could add +2d6 of SA damage, once, but that's not what makes the interaction crazy.