Syndra's primary concerns are long range champions, other control mages, rapid and low cooldown mobility, then anyone who can defensively itemize against her. Tanks and fighters are really good, for example. High mobility champions are, literally by design, meant to lose to her ultimate (because point+click counters mobility). Not in a single spell rotation sort of way, but the ones you named (LeBlanc, Ahri, etcetra) should never be comfortable being near her. Their opportune times to engage are in low sphere counts, any wasted Scatters the Weak, and when early rush defensive itemization (e.g, Abyssal Scepter) is strongest. If their approach is literally 'dodge every skillshot and win by default', I'd consider that a problem on their design rather than hers.
If Syndra manages to survive through her slow charge up windows, then her ultimate is dangerous to them. Unlike, say, Veigar, whose old incarnation was literally a measure of his AP value rather than a time one. At which point, it comes down to whose still in the danger zone and hasn't disengage yet, really. Itemization is still noteworthy here as playing pure glass cannon against her is also playing into her Burst strength ... because Burst counters pure glass cannons.
(Coincidentally this is why we largely see Burst and Point+Click damage tied together in League's game design, and AOE damage doesn't really reach the same individual thresholds.)
Assassins are funny for her in specific because she's equipped to go head-to-head with them and win as much as she can lose. That particular match up is extremely volatile, and individual decisions rack up rather quickly, where as someone like a Tank can walk all over her with 2 items in the early/mid game.
I'd ponder if it's a problem more inherent to the Assassin game design pattern, though. They're in a fundamentally 'always losing' position and have to take incredible risks to stay game relevant. Virtually every other class is comfortable in a fall back pattern of some kind, so they're not pressured into unfavorable situations to try and out play.