Pity the others, particularly MizuYuuki and Sasaki Kojiro, have departed as they had a better understanding of the raw mechanics than I did. From what I remember, the BFN "fear-factor" was similar to the fear-factor related to Warrior Monks, but I can't remember if it was a direct morale effect or something more specific to these two unit types.

I was disappointed in BFN performance too when mixed in with other units in a stack. Individually they are very powerful, but there are only twelve in a unit and they are EXPENSIVE both in koku and time to produce. I did have situational success hiding them and then using the classic Mongol "feigned retreat" tactic with my other "visible" units (cavalry usually, for speed and antagonization--got a thing for horsies, lol), the goal being to get the enemy army's general to impetuously charge after them--right into the hidden BFN cluster. If a general is killed there is a well-documented big morale hit for the entire enemy army for a short time, and a smaller lingering morale penalty thereafter. BFNs will make short work of a hatamoto unit provided they catch them by surprise and unsupported.

This kind of tactic works well at a bridge--place your BFN near the mouth of the bridge but far enough away that you can choose when to use them to choke the bridge on your side. Don't move them at all until you want them to attack. No point in surprising initial ashigaru fodder and bait, but when the boss tries to charge across ....

But as I mentioned, I found they work best in an exclusively BFN attack force. In the scenario I mentioned, the Oda usually attack Mino very soon after game start. In 1580 you start with nine BFN units as Tokugawa. On the turn that the Oda leave Owari except for a small garrison, I attack with the BFNs only. You'll have to attack via the 3D battlefield (not auto-resolve). Owari is a two bridge province so I split them and cross each bridge, the intention to bracket the defending units. But very consistently, provided there are 300 or less defenders, they will turn and flee the field as the BFNs approach. I just chase them off--BFNs way too expensive to waste on cheap standard units. I then move the big regular army stack waiting in Mikawa to Owari (the original Hizen stack) as the BFN cannot effectively defend the province from certain counterattack. Usually the Oda will retaliate immediately, but are easily devastated when they attack my full stack in a double-bridge province. Given this is early in the game, taking Owari and crushing the avenging army will cripple the Oda henceforth and allow you to focus on the Takeda, Hojo, and Uesugi. The Takeda are very dangerous early on in this scenario ....

You can use this tactic situationally in other provinces too, just need a strong BFN stack (9 is minimum; I slowly add to that until it's a complete stack of 16) with a regular strong and balanced army stack next door.

BFN special forces can also attack poorly defended ports this way--a great way to make koku, which is very important for Tokugawa in 1580 as his province income is limited. Send an emissary, shinobi, or ninja to the port, verify the nature of the small defense garrison, verify that a large defensive enemy stack cannot move to the target port province in one turn to defend against the BFNs, and then attack with the BFNs. The goal is to capture the province, raze everything for koku, and leave immediately--preferably to the next lucrative poorly defended port target. Might want to leave the port so you can come back later after they've rebuilt :).

Many players felt this was an exploitive tactic given the simplified STW mechanics for port-to-port attack. My take is: in single-player do what you want. 1580 Toku is hard, so I didn't feel too bad when circumstances presented the option.

Anyway, I'm ramblin' on. Though you are playing Imagawa, the starting scenario is similar and you have eventual access to BFNs. You could always experiment with the first ten turns of 1580. I actually spent a lot of time playing the first ten turns of most the factions/campaigns, trying different openings (like chess :). It's the only faction/campaign that starts with already trained BFNs available to the player.

I enjoyed watching the video you linked. Thanks for the post.