Why is Pykes combo so easy to hit?
It's not. Plenty of meta champions (Lucian, Vayne, Ezreal, Yasuo, Riven, etc.) have the mobility required to dodge every single ability of Pyke's since they're all telegraphed and delayed. Not to mention every champion pretty much ever takes flash, botlaners have a support to peel Pyke off of them or heal off his damage, and simple MS boosts are enough to straight up walk out of his E.
Thresh and blitz (we are leaving out naut, because that poor champion is never played) have to zone out the bush or the side of the wave to get a chance to hook someone,
And Pyke has that same problem? He doesn't get a free pass on his hook and it's easily the most telegraphed ability out of all the supports.
To stay on the topic of comparing Blitz/Thresh to Pyke: Thresh's hook has a 0.5 second delay and quickly travels 1,100 units. Pyke's (max range) hook takes a full three seconds to charge up and at max charge it just barely reaches 1,100 units, the same as Thresh.
Pyke has the added bonus of being able to move around and shorten his hook, but at the end of the day they each have pros and cons. Thresh's team engage is much better, he has much more consistent peel, and has the ability to tank.
Pyke can invis and not even have to work out zoning a lane, because he can just appear where he wants.
He still has a detection radius (quite a large one at that) and he has a massive indicator when you're within a certain range of him, even when you're in ult range. It's not just "Press W and appear on the other side of the map for a pentakill", a good team will keep a lock on Pyke's general location and protect people who are in ult range.
If he does get walluped with a skill shot, he gets all that hp back anyways.
His regeneration scales, he doesn't regain 100% of his lost HP. He also has the added bonus of dying within less than a second if he makes one misstep.
Pyke is not risk-free.
Compare to leona. She goes in, she will die if anything in the 2v2 goes wrong.
Compare to Alistair, who keeps getting nerfed and is never played low elo now. Goes in, dies if he doesnt immediately ult.
I don't know why you think Pyke wouldn't die if something went wrong in a 2v2.
Let's say I'm fighting a Lucian/Brand bot lane. I charge up my hook, Brand has enough time to E/Q me and burst me for nearly all of my health if I don't E away fast enough. If I landed the hook and followed up with my E, Lucian will E out of it, then they'll burst me down.
He's too easy to play. His ult's too fun to want to mess around with and nerf, but could we nerf how EASY it is for him to full combo people with no worry about being punished for doing so?
No, he's not that easy to consistently perform well with. One of his strongest counters is insanely strong right now (Sona), almost everyone and their mother has a dash or blink in their kit, and there will usually be one or two ways to heal out of Pyke's ult threshold, shutting him down for the rest of the teamfight. To add to that, an Annie or even Morgana has enough damage to burst him down instantly since he can't build health or in most scenarios, resistances.