Coming from both Aikido and FMA, IMHO it all depends on who is attacking and who who is disarming. I have experienced it working and I have experienced it failing. Its not the system, its the person. If you have concerns, cross train.
Quite correct. I think it is a mistake to blanket ALL Aikido as being ineffective against certain threats or to say that ALL FMA will come out on top in a knife encounter. It really comes down to the individual imho.
We also can't assume a person's training methods in detail from the name of the general method he studies imho. That creates a large opening for surprise in my experience. Underestimate no-one.