Leblanc top is good

I don't think she can burst kill a tank once they get their first negatron.

I have gone against ryze top many times and lb wrecks him

I'd argue that there is more than Ryze top lane and that doesn't necessarily make her a good top laner as much as it makes her a potential counter to Ryze.

leblanc can't burst someone with a banshees or spirit visage. she's an assassin meant to target squishy champions. you'd have a strong early game in top lane but you would be quickly outscaled by anyone who itemized properly against you.

she also does poor damage to towers so their top laner can roam freely without worrying about your split push.