I'm training IKO Kyokushin Karate right now. I don't know if people consider the Kyokushin way of training to be brutal or not, but personally, I found it excruciating as a beginner. Overtime though, my body got used to being banged up after a several rounds of sparring, strength training, and core training. Now, I don't feel as much pain when I fight, I heal much quicker, and my body doesn't swell like it used to.
The Kyokushin methods of training vary from dojo to dojo. Since many people take up Kyokushin Karate for recreation in the United States, many dojos do not push elderly couples, young kids, and weaker students to do anything that they can't. For the students that want to take their skills to the next level, my branch in the Greater Los Angeles area has setup fighter classes which are recommended for blue belts and above.
A fighter's regular training routine within a two-hour period and 1 two-minute break could consist of 10 one-minute rounds on the human shield, pad work, several minutes of jump rope, various strength exercises, various core exercises, and several rounds of live sparring . Contrary to popular myth, we can spar with gloves, shinguards, etc. And freshmen, such as myself, are also required to fight with them on during competition. But we never have them on during pad, heavy bag, and human shield work. Fighter classes do not require dogi; shorts and t-shirt will do.
It is traditional for karatekas to train on wood or concrete flooring. But most Kyokushin dojos have lined their floors with foam mats because sweeping is one of the most efficient ways of earning wazari in competition..
There are aspects of Kyokushin Karate that would work for MMA and other aspects that won't. As it was said, we fight at a strange distance compared to other types of fighting since we don't allow any grabbing whatsoever and punches to the face. Likewise, although I'm not entirely proficient with MMA and Muay Thai fight strategies, many fighters of different styles would be fish out of water in karate competition.