This is only my opinion, others may disagree;
Traditional is founded in tried and proven methods of martial arts skill development. It is not easy to learn, takes a long time but has lasting and profound results. This is the method that I have followed and it has served me well.
"Modern" Mixed martial arts are a quick fix mix mash of borrowed and favored techniques. Depending on the fighter they can be of high or low effectiveness.
Traditional training forces you to pass through the various levels of skills development under the guidance of a trusted mentor, whereas the Modern practitioner usually under a collection of different instructors might pick, chose and, or even skip certain phases they don’t understand or do not care to learn.
(Experience will show the error or gain from these choices….)
Bruce Lee for instance skipped over many aspects that he didn’t care for in his traditional experience. Personally I think if he had survived his Jeet Kun Do would be a great deal different combined with his more mature and wise self.
All martial art has many levels of training. IMHO besides internalization of the basic root skills the most important stage is self awareness training. The Book of War emphasizes that in order to be invincible, one has to first know thyself and also know your enemy. Knowing your enemy depends on your experience and also your enemy. But, knowing yourself is something that is under ones absolute control and it would be foolish to not capitalize on it. The training of self awareness can be developed via meditation, self reflection, knowledge and theory studies, and internalizing ones practices. The outcome of this stage of training will determine whether the practitioner becomes an enlightened martial artist, or just another fighter.
I have seen many enlightened martial artist arise from the traditional path. I have yet to see even one enlightened martial artist arise from the modern path…