So, first, let's look at his actual achievements. Aside from being a seventh dan/degree black belt in Aikido, he also possesses black belts at other martial arts such as Judo, Kendo, and Karate. Though videos of these aren't available and are very rare.
People also try to believe what they read/heard. They tend to believe that Steven Seagal was choked out by Gene Lebell, which I'm not saying is unlikely to happen, but unless there's actually solid proof, I think I have every right to postpone my judgement on whether this story is real.
Now, for Steven Seagal himself, from the videos, we can now know that Steven Seagal is legit. But some people, who think otherwise are just close-minded. Just because they've seen him in bad movies (And even good ones) doesn't mean they can right out say hes a fake. Another thing why people choose not to think of him as a real martial artists is that Aikido, a martial art created by Morihei Ueshiba, (who is also a successful martial artist in his own right.) is said to be "Fake" now, this is just an uneducated statement. Just because it seems fake to some people doesn't mean it is. It's was/is used to train the United Nation Department of Safety and Security (U.N.D.S.S) and is used in some branches in law enforcement.
And, another thing, yes Aikido has some downfalls, but non-resisting or resisting opponents can get affected by the techniques. Honestly, not all techniques will be applicable, but this isn't the first and only martial art to have some techniques ineffective to some circumstances. And if you bring Bruce Lee in this thread, let's not forget his quotes.
"I fear not the man who has practiced a ten thousand kicks once, but I fear the man who has practiced one kick ten thousand times."
I also remember a quote where it Bruce Lee said that take what you know and use it. The rest is useless. Or something alone those lines.
And if you still refuse to acknowledge Aikido as a real martial art, how about Judo and Karate? Maybe Kendo?
Besides, Steven Seagal is still a six foot four behemoth.