The biggest problem I have seen with krav maga schools (and other street/self defense combative stuff) is that they teach street applications of techniques but often utterly fail to teach the basic technique soundly in the first place. A simple but, from what I have seen all to common example, is a basic punch. You are not going to punch very hard unless you have sound punching technique. I'm yet to meet anyone that has trained exclusively krav maga (or the like) that can throw half decent punches.
Also like anything else the quality of instruction is key. I know and have trained with or around (one what actually train with us) two different krav instructors and their students regularly. One is pretty good the other is laughably bad, and chuck full of BS and bravado to boot.
In my experience, if you want to be able to fight you need three basic skill sets: stand up striking, wrestling, and ground skills. There are sport applications and street applications of each. There are also techniques that are only suited for one environment or the another. That said the basic underlying skills are the same. Once you can the base skills you can focus on street application. You can also expand them out into other areas, training retention of firearms, etc. You will have a good sense of what is practical and what is BS that is highly unlikely to work.
The things worth learning are IMHO the basics of Muay thai/boxing for stand up and footwork (one of the most underestimated parts of fighting). Some wrestling and judo. Some basic ground fighting (sweeps and getting up in particular for the street). If you're interest is in self defense you can focus on that application after you have the basic skills. However, the basics are the basics and transition over very well. I personally use a very different approach and strategy for sport fighting and ring fighting than I would for self defense.
If you want to be able to fight train in something where you are actually fighting. If you aren't getting hit and hitting you are never going to learn how to fight. Also, honestly some people just are not fighters. They could train with anyone from now until the end of time and they would get beat down the first time they ran into real trouble. It is better you learn that, or other limitations you have in training and not when it really counts.
Additionally, fighting is a physical activity. Strength and conditioning actually go a very very long ways in a fight.
I'm not disparaging taekwondo when I say it is not the best art for sd.
I'll be blunt TKD (and a lot of other stuff) is horrible for self defense. Any sport that disallows punching in the face and trains with their hands on their hips is not good for real fighting. Most traditional martial arts are equally horrible for self defense/real fighting. These arts teach skills developed around a very artificial set of rules and skills that have very little practical application beyond them and that for the most part translate very poorly to a real fight/self defense. I used to try to be nicer about it but truth is truth and trying to pussy foot around the truth is actually doing people a disservice.