Okay, speaking from a Main's point of view.
His Q is indeed strong Level 1 to 9. But he falls off really hard if he isn't ahead(i.e getting fed enough to buy a 1.5 item known as triforce).
So why is he strong ? Because most Jungler being played are Duelists. R is bad against Duelist. R is good against tanks, but it is abysmally weak as it doesn't do % damage. Also we are in a "One-Shot meta", that is very early game oriented. About getting first towers at 10 and rotating to get herald and snowballing from there.
He is easily countered by Autoattack blockers and CC. If you play Jax, Nocturne, Pantheon, Rhaast or anything thereof he is basically a sitting duck. He is weak against scaling tops(Trynd, Fiora, Irelia, Camille and etc), and he is super weak vs an enchanter mage like Lulu and Janna with a hyper carry. Perma slows and constant cc fucks him, even if he has 50% tenacity from L:T, Unflinching and Mercs.
He doesn't scale well late. His Q is single target damage. He has no gap closer. And doesn't scale or synergizes with many items(as he used to, due to his items being reworked or removed(Old Bork, Zephyr)).
I sweat beads if I don't have an inhibitor tower before 25 minutes, as the Grouping phase is the worst performing time for him.