syndras issue is that she suffers from cast times and animation difficulties. her range is average when compared to vel koz, ziggs, xerath. her only redeeming quality is her long range AoE stun. which takes 2 abilities, basically meaning it has to be set up under just the right conditions so she can become able to land the rest of her abilities safely.
all in all, if she gets a damage buff she becomes able to combo 100-0 again with just one rotation throughout many stages of the game, which riot doesnt want her to be able to do (god forbid a mage lands all her skill shots)
and they dont want to increase her range without making her a huge lane bully.
so that leaves next to nothing to increase her power level. other than ofc, reducing cast times on her which would again, probably break her and require lots of fine tuning to see what works.
shes just impossible to balance. either OP, or UP. no balance.