Should I be worried that aikido in general is not effective for unarmed taking away tanto, or believe in aikido and that it is powerful enough to take away tanto safely, just I haven't seen it yet?
It all comes down to the person. Anyone with a knife is dangerous, even a young kid. Disarming a trained attacker with a knife would be almost impossible without getting cut.
Best thing to do is avoid the situation. Aikido training with knives teaches principals but I personally don't think they are meant for actual knife disarming.
I know everyone has an opinion on this but I can almost guarantee that if I had a knife and you tried to take disarm me, you will get cut no matter what Aikido rank or style you are.