If Syndra is missing everything and then deleting her with her ultimate alone then one of following two scenarios has happened:
1.) Syndra missed her stuff and only got a few spheres out and still almost deleted you. For this scenario to happen she has to be fed to ungodly levels. And if that Syndra happens to be the one who misses all her abilities often like you mentioned then how the hell did you manage to feed her? And we are not talking about 1 or 2 lucky kills she gets with a jungler gank, we are talking about much bigger advantage,
2.) Syndra missed her stuff until she got 7 spheres out and then deleted you. In this scenario, you let her spam her spells for at least 10.5 seconds (the amount of time she needs to get 7 spheres with 40% CDR if she spams them on cooldown). And honestly, if you are dumb enough to not punish her for dancing around you doing effectively nothing for 10+ seconds your nexus should immediately explode from sheer stupidity and not just your lifebar.
And it's not like she still has 3 sphere grab anymore which used to make 7-spheres ult easy to pull, so you don't even have that for an excuse.