Back in the day, I started training in aiki-jujutsu to supplement my "3K" karate training. Hell, we all did 3K karate back in the day, that was the norm! I knew there was something missing in it but for the life of me I couldn't figure it out. There was an aiki-jujutsu dojo near my house. The sensei, Bernie Lau, only taught police officers. I humbled myself and begged him to take me in as a student and then my journey began. After training in aiki-jujutsu my understanding of some of the movements in kata became a lot more clear and I was able to create some interesting bunkais, something that was verboten in 3K karate. Aiki-jujutsu and aikido, in general, has problems with street effectiveness just like karate. It's not that the art is not effective, it is the application of the art that is flawed. That is the difference between karate-do and karate-jutsu. I occasionally watch aiki-jujutsu videos to see different applications for karate techniques and one can find a lot of hidden kata applications by doing that.
I recently came across some videos done by an aikido sensei named Lenny Sly. This guy is really good and he has figured out how to take all the fluff (earth biscuit and Moon Beam BS) out of aikido and aiki-jujutsu in order to make it more effective on the street. Some people might be turned off by his attitude. I am not, because I understand exactly where he is coming from. It's not ego, it's just a matter of making an ancient art work in the modern world. A lot of people can't do that. Karate is the same way. Here is a link to sensei Sly's website (YouTube page). Add this stuff to your karate. When you're fighting from the clinch, as most fights do, you can see that this is a force multiplier for your self defense karate. I see aiki-jujutsu more as another aspect of karate than a stand-alone art. I say this because it is also "Empty Hand!" Let me know what you think of this. I find the joint locking stuff fascinating. It is a force multiplier when added to karate. The 2 arts compliment each other.