The mobility specials in a free moving engine are way too good, as many have said.
To combat them you generally need to give a lot of respect and think 2-3 steps ahead of the opponent. It sucks because against the best characters, their frame data/neutral state is so strong that you can't just play a game where you're able to react to those movement specials while covering other things.
Anyway.
ZSS isn't easy.
I don't think she universally beats everyone. She's got a lot of potential though.
BUT.
her neutral b laser is not god tier. It isn't "safe" on shield or a frame trap unless charged (a lot), you can definitely take a positional advantage gain against her if you're shielding it properly. Most characters can completely out prioritise it with their aerials and smack her (which is an interesting nuance of the sheik MU where her fair isn't doing enough damage to beat it out, so ZSS gets guaranteed clashing stun Up-B's)
Her down B is quite good but definitely not disgusting like bouncing fish or quick attack at all, not even close. I'm not sure how the invincibility works with the move (perhaps moving the stick for 'rotations' is what causes it or something?), but it's somewhat consistent for a things like thunder, all of diddy's aerials, etc etc to just 'win' through the spin. It can be used aggressively but it's high risk / high reward.
It's one of the best moves in double though, holy crap. ZSS can literally be controlling two fights on opposite sides of the stage at the same time better than most (if not everyone) because of how destructive ground stomps / meteorish footstools / super kicks can be when covered by your partner and the speed
Up-B is very powerful, but somewhat like Ness if you're ramboing into her off stage and can pop out with DI you'll kill her in horizontal scenarios. If she loses down-b she's in a lot of trouble usually, but it isn't too bad if you have your mid air jump, better players expect the mid air jump option too though. It roofying and killing early is a mixture of people not having optimal DI (you want to go horizontally) and not respecting that you can't DI/shuffle out of it if ZSS picks 1 of the 3 angles properly a lot of the time (unless you're little mac).
Her challenging (not necessarily losing) match ups include
Possibly/probably loses to
; Greninja seems hard too? Olimar?
*cough Ganondorf and Link do pretty well cough*
Anyway a point on ZSS is that she still struggles against smart shielding at mid-range. Don't over shield, that's silly (she's fine at waiting it out, and that's actually a really good situation for her a lot of the time when she reads shields, she can stand and react eventually
). But unless neutral-b is charged it's pretty easy to negate it as a pressure tool, if you're a short character nair, fair, uair, bair, zair are no longer immediate or sometimes ever usable options (rar bair does swipe short characters sometimes though I should check the heights sometime).
ZSS can get grabs in this game better than in Brawl imo due to the shield changes, but at the same time I don't think people should be getting grabbed after any form of shield pressure; don't do silly rolls or jumps.
Beyond Up-B set ups or OoS punishes (based 4 frames), she can really struggle to kill, often only being able to fish for back airs. Back air is long range and generally safe but it's only out for 2 frames and can be crouched under. If people get consistent/better at shuffling out of Up-B she'll be kinda screwed tbqh. But i'm in the boat that the right position + right angle of up-b guarantees it 90% of the time.