Is Cassandra's manual node addition/removal tasks tedious and practical in a large cluster?
There are mentions of clusters with size 300-400 &amp; more machines.  But plenty of tools are provided with Cassandra that could help with automation of deployment process.  Plus you can find a plenty of recipes for Ansible (1, 2, ...), or some other automation frameworks.

More work could be required for other admin tasks, like, making sure that repairs are running, etc.  But people are doing it, and there are "3rd-party" tools for them as well.