Ok, so I play both Assassin (45), KFM (43) and I like them both. I'm not sure at pro play but at gold/plat range, the matchup as a KFM is not that hard, you just have to watch carefully to your opponent moves.
1. Is the Sin an easy and OP class?
At low levels bronze, silver... YES. RB+F spam while invis is so strong. But at higher levels it's not like that. You need to know how to combo (as a sin) or you are dead in most of the matchups. More or less like KFM, but instead of 3RF, the web+moths combo (which is harder for me).
2. Is a difficult matchup KFM vs Sin? (from the KFM perspective)
From my experience, Assassin has more tools than KFM, but is a bit more unforgiving. As Orihalco says, as a sin you need to control the flow of the match or you are dead.
When I play as KFM vs sin, they are not a big concern for me. They initiate with Run+RB? (Most of medium level sins do that), you get them out of stealth with a Rising Dragon (Z) and if u miss, tab your ice protection or leg sweep (3).
While they are in stealth: are they far away? prepare your SS to iframe their jump or their shadow step/shuriken. are they nearby? take them out of stealth with your aoes.
Are they visible and ranged? Most of the times block their body swap.
Tbh, I think is a pretty fun matchup: as a KFM you keep a defensive position and you need 1-3 combos to take sin down. As a SIN you are the agressor and you need to poke constantly to find the KFM's weakness.
3. To finish: about Sin stealth.
I think Sin's stealth is mostly a "preassure factor" when you don't fully understand the sin class. So you get overwhelmed just by the fact that you cannot see your opponent. However, once you understand what they can do from stealth, it's not that bad.
Sorry If my english and my game knowledge sucks. Just wanted to share my thoughts on the post as a Sin/KFM player. Have fun!