It is simple.
You train in Bujinkan to be good at Bujinkan.
You train in Self Defence to be good at Self Defence.
You train in Combat to be good at Combat.
Bujinkan per se, is not Self Defence or Combat, and as has been said, it will not make a 'fighter' out of anyone who does not already have the mindset and a bit of experience to be one.
But you can train to get your Self Defence in top notch condition, and you can train to be safer on the streets of Rio or Kabul, or in law enforcement etc, if you integrate Bujinkan into your self defence and combat training.
Lets face it, if you want to send 100 young men to war, do you send them to a local Bujinka dojo or an army camp? If you want to send 100 law enforcement officers out on the streets do you send them to a dojo or for police training? And if you want your sister to be safe at University but she only wants to train in condensed self defence strategies for a couple of months and not spend years perfecting Kihon Happo and Gyokko Ryu kata, do you really send her to what generally constitutes a Bujinkan dojo?
Lots of people use Bujinkan for combat, law enforcement, self defence, bodyguarding, even sports martial arts in some cases, but the Bujinkan is only an aspect of what they do, even if it is the underlying core.
Look at youtube, look at all the black belts, do you really think they are all real mean tough streetfighters? Look at their students, doing floppy complacent, compliant training with friends they can trust, a university hobby, no different to football (and probably less use in a tight spot).
The Bujinkan is a fantastic thing. But if you want to use it for combat and real fighting, you need to train as though you are going to do so, or you will be in a real state of shock and a lot of pain very quickly when you find yourself in a situation where you need to protect yourself and others from harm.
We are told to beware of wolves in sheeps clothing, but we should be far more concerned about sheep in wolves clothing, and should not become such a thing ourselves.