Let's stop pretending he's a "bad" champion even if he's somewhat basic in his kit. He has an over 60% win rate early and unlike J4, Rek'Sai, Pantheon, Olaf etc., his win rate is still fantastic even late game. He's hovering 54% past 40 minutes and his mid game win rate is good too. There is no part of his power curve where he's actually doing poorly (unlike say Amumu who starts with a terrible early win rate). This is even more impressive considering how varied his builds and runes are. He's performing this well despite a lot of people picking suboptimal stuff at a fairly high frequency. This champion is actually amazing and I'd argue overtuned in a SQ environment (perhaps underestimated in competitive). He has had the highest or near highest win rate almost every patch this season.
In fact, he is more oppressive vs weaker early game junglers than anything else I've faced and doesn't even really get outscaled. Yes, his teamfighting may be bad but he can create a lot of disruption, split, take objectives solo and is a menace in skirmishing. I don't know why he's still "sleeper" or why no one talks about him.
His ganks may not be the most powerful but there's always going to be someone overextended in SQ and he can chase them down for a basically guaranteed kill. Even if the lanes play super safe, that means their jungler has no priority and can't safely invade or farm his own camps. They will be isolated vs Udyr because lanes can't roam to help when their wave is almost at turret. Many junglers can't even run if Udyr sees them and apart from maybe a level 2 Pantheon, I'm not sure if anyone can actually duel him.
For people saying that he's bad in high ELO, the current rank 7 (PantsAreDragon) has more Udyr games than any other champion and with an insane 68% win rate. Udyr is not fine and needs slight early game nerfs if he's going to still be pretty solid late game. Even something like -3 AD may do something.
BTW: I realize that Vi is statistically doing just as well and has virtually no losing matchups. She even has twice the play rate. The reason I'm so annoyed by Udyr is due to his insane level 2. He can shut me out of the game on weaker picks. Vi at least needs level 3 to be relevant and a lot of her win rate is due to the utiltiy from her R, Q and even the shred on W. She's not usually walking into your jungle early and forcing you out and she'd be taking a serious risk doing it. Udyr is so fast that he doesn't even get punished for it. I think Udyr is much stronger than Vi in the hands of an expert jungler who knows how to abuse him.