Sadako is actually the second best stealth killer right now... which is still quite weak.

I've seen a lot of people in this forum saying Sadako is the worst killer in the game. I must say she is not THAT bad. Actually, she is the second strongest stealth killer right now, falling just behind Wraith in that regard. Her stealth ability is far better than Pig and Ghostface's, since she doesn't slow down, can't be broken out and also can't be seen from too far. It can also help her a little bit on mindgaming loops. Not much, but better than Pig's charge attack. It also allow that to ignore colision, walk right through survivors and avoid bodyblocking. While she has no instadown like Ghostface or Myers, she has her Projection ability, similar to Freddy's teleport, but nearly instant. Which is great for applying pressure, specially on huge maps like Mother's Dwelling, Azarov and Eyrie of Crows (the last one being huge when it comes in actually used distance, occupied by loop tiles and gens you need to walk to). The only thing preventing her from being the best stealth killer is the fact that each TV turns off after being used and can also be turned off by survivors, which means Wraith's speed is more reliable.

However, she is still too weak. And why is that? Well, because STEALTH KILLERS, in general, ARE REALLY WEAK in this game. Most maps have wide open areas with not enough tall objects for the killer to hind behind, and many gens are in areas with no tall objects at all, so survivors working on them just can't be sneaked upon. Doesn't matter how sneaky you play, good survivors just can't be caught of guard in many areas (And here's why I will defend indoor maps to my dying breath). And because survivors have third person view and the killer has first person, sometimes you are spotted by them before realizing they can see you. Also, stealth killers don't have strong abilities in chase, their powers being better to approach and get hits before the chase starts, and these hits can still be robbed away by Sprint Birst or Dead Hard sometimes. That's why Pig and Myers are among the weakest killers in the game. Ghostface is at least getting some buffs but still isn't going to be that strong, and Wraith, while a little better, is still quite weak against good survivors.

Anyway, the way their powers work tends to be good for hit and run playstiles and sinergize well with perks like A Nurse's Calling and Sloppy Butcher to sneak up on healing survivors. Of course, hit and run tactics and Sloppy are weaker now thanks to CoH. Stealth killers were already weak, but CoH makes them even weaker. Sadako is specially hurt by that because her power works a lot with hit and run.

TL;DR: Sadako is better than most stealth killers, but being a stealth killer she is still weak. Map design and CoH have a huge impact on that.