Says me I guess,I don't know if others would agree but the way I see it Leon is Yuri's overall greatest threat. A good leon can snatch you out of the air no matter your position with tongue
His tongue can only be aimed in eight directions. If he's cloaked it doesn't matter as much, but I rarely stay still with Yuri at all, especially with a Leon after me. If he isn't cloaked it's usually obvious when he's trying to get into position to grab you. He also has lousy control in the air, which is basically where Yuri lives.
and prevent any hopes of escape with slow,
Once again, Electrifier is not inescapable. Early game I break out most of the time (if I get killed it's usually to the immobilize on slow bug or a team ambush). When Time warp is upgraded enough I can hunt down and kill them with very little trouble. Either way, it's less of a threat than a Dashtornado™ from Froggy or a Lonestar knockback combo.
his clones can also destroy his mines and knock him out of the air.
Minesweeping isn't exactly unique to Leon, and it takes a long time to clear them with clones unless you put them all in one spot. And I really don't see why knocking someone down with clones would be an issue.
Stealth allows him to get the jump on him much better than lonestar and he's more difficult to flee from.
If sneak attacks are a major problem you might want to work on positioning. Leon usually has a limited number of approach options, and a lot of them can be cut off with mines. On top of that, almost every Leon has habits that make it even easier to tell where they're coming from.
As for fleeing, no. Lonestar can chain knockbacks faster than Yuri can recover, and Froggy can catch him (or cut him off) even with Time warp/Flash running unless Yuri has a big head start.
Lonestar is still fearsome but as long as you stay up high he's not too big a problem since he can't force you to come out of your safe zone without getting behind you with bull.
For Leon to reach higher than Lonestar or Froggy (if he can at all) would require tongue grabbing you from off screen, which doesn't really accomplish anything unless there are others on his team waiting for you to get reeled in. If he's lucky he might land one hit before he falls and you can fly again, but that's about it. Lonestar can stick to you with triple jump and it will take a lot longer to recover if you get hit by a missile. Froggy can Splash dash and drop him all the way to the ground.
Also, if you're in a spot where nothing can reach you, you're little to no use to your team, and not a threat to the other one.