- "Pretend" martial arts, that exist mostly for show, but which are pretty much ineffective in practical situations.
- "Sport" martial arts, which are often "simplified" versions of past martial arts, distilled as a sport (rather than an actual self-defense mechanism in practical situations).
- "Practical" martial arts, which do not exist for sport (and often even abhor participation in competitive events), but which nevertheless are too "traditional" and too rule-bound to be all that effective.
- "Dirty fighting". I'm not using this as a derogatory term, but as a term to mean "doesn't obey any rules".
The first kind can immediately be discarded. There exist different grades of it, the worst ones being outright in the realm of fraud. The less obnoxious ones may be taught genuinely, but are still completely ineffective for self-defense often even against inexperienced assailants. The most skillful variants are very beautiful to watch, but they are pretty much just choreographed gymnastics and little else. They are still pretty much useless in real situations, unless you get very lucky.
The second kind exist (and are often outright "marketed" as) just for sport. It's a form of exercise, and there may be competitions, which are run under very strict rules and limitations. They might work in some cases in real self-defense situations, but usually only against unaware inexperienced assailants. Even then, perhaps not.
The third kind can be more effective and brutal (which is why they often abhor competitions). They may often work very effectively against unaware assailants, when performed by an experienced practitioner. However, even this kind may be too stuck on those certain "practical situations" that you may encounter in real life (such as someone grabbing your wrist, or trying to punch you in the face), and too little (if at all) on opponents using the final category, ie. "dirty" fighting, which is the next category:
"Dirty fighters" are experienced "street fighters" that don't care about rules. They just use the most effective way of beating you up, which is often grappling and floor wrestling. This is the essence of so-called "mixed martial arts", or MMA, and it's brutally effective, and it (pardon the pun) mops the floor with all of the above pretty much every time. You may have decades of experience in punching, kicking, getting out of grabs, and getting your opponent to the floor when both are standing, but when a "dirty fighter" grapples you to the ground and starts punching your face, that's usually not something you have much experience about, in any of those above variants. And they are very effective at that; if you are a practitioner of one of those "practical" martial arts you might think you can avoid it, but believe me, you won't. Those fancier variants don't do much of efficient floor wrestling (if they do, well... then they are pretty much MMA.) Perhaps the only way to avoid it is if you can run faster than the assailant, and you can escape (something that, in fact, and in fairness, some of those other martial arts schools teach as the most effective and most important tactic.)
There are countless videos out there where a martial arts master of some sort has agreed (for some reason, perhaps pride) to fight an experienced MMA fighter, and the master is inevitably beaten. All those fancy techniques just don't work against a hardened and experienced floor-wrestler. The MMA techniques may not look nice and fancy, but they are brutally effective.
This was made quite clear in the first UFC tournament ever organized. Back then, when the whole idea was new, people didn't know what to expect, so many people from varied martial arts genres participated. The floor-wrestlers dominated the tournament, and were absolutely brutal. The other competitors simply had no chance. (It was made even more brutal by the fact that back then there were significantly less strict rules than today. For example today it's forbidden to hit an opponent on the back of their head; back then almost everything was permissible.)
That being said, there's one thing I really dislike about MMA as a sport (rather than a self-defense skill.) I repeat: The sport of MMA fighting.
In the vast majority of fighting sports there's one extremely important core rule, one of the core tenets of sportsmanship: Never hit a fallen opponent. In fact, in most of those sports doing so is considered utterly unsportsmanlike and despicable, and often carries heavy penalties, including disqualification. Hitting a fallen opponent is considered cowardly and opportunistic.
Yet this is permissible and expected in MMA. Your opponent falls, you immediately start hitting them as fast as you can. I don't like it. It just feels so cowardly.