No assassin ever should have winrate above 49%
At best they should float between 49-50% but at this point one should be having an eye on them and nerf hammer in his hand.
Assassins supposed to have high skill-cap. Necessity of putting themselves in dangerous spots or being able to find right timing for going in should require good game knowledge and mechanics to be successful. Which first of all is impossible for 70% of player base. At least as often as it should be to make reliable positive impact.
If they have 50% or close winrate it means that assassins are too forgiving and too rewarding. Too safe and too impactful.
You should never see assassin as first pick in meta where squishiest thing on enemy team might end up being support Alistar or Rakan. Or some Darius that decided to get one offensive item too many.
Fight me.