Hey! Pyke main here.
PASSIVE: GIFT OF THE DROWNED ONES
SO HE GETS GAREN'S PASSIVE, AND GETS SCALING DAMAGE
While I personally think his grey health needs to scale more with AD and be nerfed overall, he has the drawback of not being able to build any HP.
Gaining AD from HP is a double edged sword as one good round of CC will instantly annihilate him. Try picking champions with reliable CC and focusing him in teamfights.
Q:
GETS BLITZ/THRESH UTILITY WITH NONE OF THE DRAWBACK IN THAT HE STILL GETS A SLOW EVEN IF HE SCREWS UP AND TAPS IT.
The drawback (and a huge one, at that) is that he has to channel his hook, unlike Thresh, Nautilus, and Blitzcrank who all have near instantaneous hooks with no windup. Pyke also doesn't have a consistent CC to land after he hits a hook like the aforementioned champions do.
Blitzcrank can quickly knock up people he pulls, Thresh can Flay the champions he pulls or use his ultimate to slow them even more, and Nautilus can use his autoattack to stun or E to slow. Pyke's dash has a huge delay and clear window that it will strike in, anyone with decent mobility will be able to escape it entirely.
It's also hard to land a hook and dash immediately after without going forwards, as dashing backwards will put you out of a fight or could make you miss your E entirely. If Pyke chooses to dash forwards, it will put him directly into the enemy team and make him very susceptible to focus.
W: GHOSTWATER DIVE
** GETS INVISIBILITY AND MOVE SPEED BUFF, SO WE HAVE A YI/EVE HYBRID NOW.**
Well firstly, there's a large indicator under you when Pyke uses Ghostwater Dive. Not only should that be a large enough indicator, Pyke can also be spotted by Control Wards, Towers, and a different indicator/sound will be used when you're in execute range.
As mentioned before, Pyke cannot build any sort of Health and building defensively pretty much gimps you now that he needs Lethality to scale with his abilities. Pyke needs a retreat spell like Ghostwater Dive to help engage and disengage, as well as proccing Gift Of the Drowned Ones. It's the same reason why Leona can use Eclipse (W), it serves to benefit his playstyle and help engage/disengage during fights.
Evelynn's stealth is also permanent with no indicator and Yi is infinitely more threatening than Pyke in most situations.
E: PHANTOM UNDERTOW
HE EVE HAS A MASSIVELY USEFUL GAP CLOSER THAT'S INSTANT AND DOES DAMAGE... WOW...
As mentioned above, using Phantom Undertow can put Pyke in some pretty nasty situations. Bait it out and capitalize on it's long cooldown or CC him with Roots, Stuns, or Suppression to get an easy kill. It's also not instant, quite often I find Thresh and Blitzcrank can knock me out of the ability altogether.
Again, as mentioned above, Pyke cannot build max health. If you want to hit this ability with nerfs, you need to give him some sort of defensive ability or mobility other than straight MS as it would gut him.
R: DEATH FROM BELOW
HE IS GAREN AND DRAVEN FULL STOP, PLUS HAS YET ANOTHER GAP CLOSER AND LUDICROUS DAMAGE.
Using this ability as a gapcloser is the same as comparing it to Yasuo's Last Breath, Akali's Perfect Execution, Camille's Hextech Ultimatum, or the multitude of other mobility abilities in the game, so I'll just leave that point out entirely. Generally he won't be using it to jump around unless the Pyke is trying to chain-ult, which can also be shut down with ease.
Firstly - stopping Death From Below is the easiest way to invalidate Pyke. There are three ways to do this: Baiting/focusing, building against it, and simply dodging it.
Baiting -
Bait Pyke's ultimate by remaining at low thresholds and when he uses the ability, pop a quick heal and waste it entirely. Even if you can't successfully bait an ult and you lose your teammate, Death From Below places Pyke directly at the center of his X, meaning you can easily focus and eliminate him now that tank builds are a thing of the past.
Building against it -
Simply build health or use armor items to counter Pyke's physical damage output. Building health reduces Pyke's ability to put you into the execute threshold, similar to Darius, Cho'gath, and Urgot. Stasis items such as Zhonya's, Stopwatch, and Guardian Angel are also very successful at baiting and wasting Pyke's ultimate.
Dodging it -
Pyke's ultimate does have a noticeable indicator. Even in teamfights, Pyke's ultimate can be seem coming from a mile away. Simple dashes like Lucian's Relentless Pursuit, Yasuo's Sweeping Blade, Vayne's Tumble, and even movespeed buffs like Youmuu's can push you over the edge of the hitbox to safety.
On top of this, you can see/hear a different indicator when you're in execute range, so if he's approaching with Ghostwater Dive you can easily prep yourself to dodge the ability.
If you're in execute range, reacting quickly to shut down his ultimate will make him much less of a threat overall.
All in all, I think Pyke is a very strong champion but he has clear weaknesses that can be capitalized on at all points of the game. Hope this helped!