Well I wouldn't know how devastating Steven Seagal is in real life
but
1) Against a common thug:
This is what I am hoping to answer, and I am tending towards yes with more realistic training.
2) Against a trained fighter:
No, but Aikido wasn't intended as such and I don't see it as a failing.
In Aikido we learn to train without resistance but we must train for a common thug's resistance (at higher grades) if we are practicing Aikido also for its self defence element.