I'm not an expert in martial arts but I always wanted to know if a offensive-based martial art, like Karate and Taekwondo, is capable of lasting or even defeating a fighter that uses Aikido. I've seen a few YouTube videos of Aikido in action and, so far, I've noticed the Aikido practitioner barely takes any hits from a fighter who used Taekwondo. In fact, almost every attempt the Taekwondo fighter made ends up with him being in a joint lock or on the ground. Like I said, I'm not an expert so I probably don't know the whole story on either fighting style. So can any of you help me answer this question?
There's no such thing as an offensive-based martial art. Only the fighter can do or be offensive things.
Karate is inanimate, and so it cannot stand, let alone stand a chance. Only the fighter can be better than another, and then only for an instance.
YouTube does not bestow wisdom, intelligence, capability, nor knowledge. It can only report what had happened at the time the video was made. The same video can be shown in different contexts and tell very different stories. Best to stay away from YouTube until you have a better understanding of martial arts in general.
But as to specifics about what you saw, the Aikido-ka didn't get hit because he employed the first rule of self-defense: "don't be there", sometimes stated as "get outta the way!". This mantra exists in all styles, whether it is for self-defense or for sport.
Don't worry about what you see on YouTube. It is no guarantee about what you will experience, or what you will be, or what you can do, or what can happen to you. YouTube shouldn't even be part of your resources until you are well under way in your training, and then only judiciously used. Therein creates many misconceptions and myths, which we martial arts - of all styles - work hard to dispell.