T9 suffers from slow input speeds. In the worst case you are required to click the same button four times for a single character.
Here is an emulator: https://www.sainsmograf.com/labs/t9-emulator/
Type `422537` to receive `hacker`. You hit `2` twice to get `ac`, not `b` - and it picks `ac` given the context.
I think the only limit is the time it takes to train a human to write using such keyboard. And perhaps more problematic how bored said human would get, and hence, give up :)