If you have Cassandra (or Cassandra-like DB) running across multiple DCs, you can definitely mitigate node, rack and even a DC failure for a 100% up time.
Just because a node or DC fails doesn't mean there is a user visible impact.
Just because a node or DC fails doesn't mean there is a user visible impact.
Your uptime is only as good as your downstream components, and no downstream component will give you 100% guarantee. You can have redundancy on top of redundancy (like space systems), but that will just stretch out your nines at best.
For the same reason your downstream components cannot guarantee you 100% uptime, you also cannot guarantee 100% uptime for a new system in isolation, for reasons the sibling comments go into.