By far the biggest issue I've seen people grieving about when it comes to Pyke is that he's actually quite underpowered...until he hits 6.
I'm probably talking more around the plat+ elo at this point, and I'm sitting at somewhat of an in-between since I (FINALLY) hit gold this season, but he seems to be a real pain in the ass in lower elo since his kit is...well, an assassin support. the people in bot lane for lower elo games usually get destroyed once Pyke starts in.
And in Higher elos, it's about the opposite: all that mobility doesn't really get him anywhere without some solid team coordination (or the enemy's lack thereof) and a well placed hook/dash. Usually, the best he can do pre-6 is just soak a few hits, maybe land a Q or two, and heal with his passive to damage soak a bit.
Where both lines intersect, however...is the level 6 powerspike.
From this point, skill and tact can be thrown out the window. That hp bar turns "kill me" red, you press R and hit that execute for stupid amounts of bonus gold for both you and your team. Now do it again, cause that other guy came back trying to save his friend and also got rekt.
Now...you and your team are getting dumb amounts of gold. Screw just staying bot, unless it's best that you just farm double kills. Try and get everyone to group - generate ALL the gold! Suddenly, in the span of 6 minutes, you went from somewhat even, to your team having a 10k gold league, and about 2/3 of that gold lead is a direct result of your ult sharing gold in teamfights.
There are very few PROFESSIONAL teams that come back from 10 deficits. I dont think I need to inform you that most non pros aren't too good at it either.
Basically...he's actually somewhat weak...but his unique combination of an ult that gives a resetting execute that gives bonus gold on each execute, and a plethora of abilities to help him keep in range of that execute (or to gtfo if he cant make one) make him quite frustrating to play against.