I would argue that it's not particularly strong. If we look to the quality source material available today (which isn't much), we're basically left with Bansenshūkai (which primarily concerns the traditions of the Iga and Kōga regions) and Shōninki (which primarily deals with the Kishū province). There's also Ninpiden of course but given how closely guarded it is and how few people have had access to it I'm a little hesitent to consider it a high quality source.

So what's in the two main sources we do have access to? A lot. Philosophy, leadership (mostly inspired by the Art of War), subterfuge, infiltration of Sengoku era fortresses and military camps, tools and weapons, espionage, how to break into a Sengoku period house, how to gather information from people without them becoming suspicious, a little light Buddhist philosophy and of course, astrology. Because if you're going to break into a Sengoku castle and murder someone, it can't hurt to have a little celestial help. I guess.

Added to this there is secondary sources about Shinobi actions and passing mentions of them in a whole host of works. We know roughly how they came to be, how they evolved from basically your garden variety of feudal Japanese criminals for hire into a more professional force. We know a little about their clan structures, and we know how it ended in the Edo period where the Shinobi had undergone a lot of changes and often served as watchmen, bodyguards and other more visible functions. Then there's a roughly 100-year gap until Ninjas make a big comeback along with a general fascination with all things Japanese.

So how does this connect to Bujinkan Ninjitsu? For the most part it doesn't. While some of what Bujinkan teaches does trace its roots back to the fighting philosophy of the Shinobi, most of it does not. Most of the ryūha that make up Bujinkan have their roots in samurai fighting techniques and all the other things we know where part of Ninja training are basically not covered at all. I know some Dojos actually actually teach things like disguises, lockpicking, philosophy and so on but most focus on the martial arts aspect and they are the only parts that the Bujinkan dojos all have in common.

TL;DR - A part of Bujinkan has its roots in a small part of what was the full skillset of a Sengoku era Shinobi.