Leblanc isn't a great champion - take it from an S2 Leblanc main who capped at diamond last season. She's not anywhere near as tricky as some other assassins with more instant kill power and much easier to kill. Her main advantages, if any, over others of her class (assassin) are her ability to stick to targets (as long as she remains just outside of their most reliable attack or ability range) and exert constant pressure without ever committing. Even when she does commit, it's just as easy to snap back out. Those are her strengths.
That also means, however, that any champion with instant, reliable damage or crowd control hard counters Leblanc. Even marksmen are no exception to this rule - if they're far enough ahead, Leblanc is nothing but easily shattered glass to them.
Champions like
are a disaster for Leblanc, because good players of such champions can instantly destroy her the moment she pops into range, even if just for a millisecond. Leblanc has no choice but to play around these champions with her gimmicky (RR) clone or avoid entering their range completely. This severely limits her effectiveness in such situations.
However, outside of these situations, she excels at delivering accurate, almost guaranteed (Q + RQ) burst damage in a 1.5 second window while maintaining extreme margins of safety relative to other assassins (if played properly).
Burst damage potential --> 7/10
Safety --> (Against non-point click enemies) 8/10, (Against Lissandra and strong point-clicks) 4/10
Utility --> 6/10 (Good-single target conditional CC)
Trickiness --> 6/10 (Sometimes works, sometimes doesn't)
Difficulty --> 10/10 (Really hard to play well in all situations, even for masters of the champion - just look at how often Faker plays Leblanc in professional games)