The purpose of a control mage (which is half of Syndra's archetype) is effective denial and control. The range of her abilities, as well as the extreme effect / cooldowns of her control tools, drive this point home. Assassins have to circumvent her tools to get in range, via whatever means specifically available to them. Otherwise, if this control pattern didn't work, the loss of power in her kit would be catastrophic enough to plummet to a failure-tier pick.
If you get hit by her E / (Q+E) combo, she successfully repelled your efforts to exploit her close range weaknesses. Disengage, or try to force the matter with more tools, as her E has a massive cooldown compared to many mobility and ranged combat tools.
If you get around it, she's in one of the worst positions imaginable and has to:
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Out damage / race to the death.
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Run away, her W may or may not help in this regard.
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Pop Summoners that may or may not matter if the enemy has theirs.
The only Assassins notably different from their previous incarnations is Talon, whose loss of his instant blink made this lane match up kind of an auto-lose for him. But, he has that issue against a lot of the mage line ups who were built to deal with the problem of his instant blink. The other Assassins maintain their efficacy as they did before, for the most part. Some are more dangerous now, while others are not.
Assassins who do not play around her, or other mage tools, will get walked all over in their lanes. I've fought plenty of pre-nerf Fizz, Talon, Zed, LeBlanac, etcetra, players who simply didn't obey the nature of the lane correctly. Or sat around and tried the same thing over and over, then complained when it didn't work. Assassins have very binary engagement conditions, but they have more than one, and waiting around is never good for them. The long game favors the mages.