My GOD! That is most inefficient unit ever. I would rather hire other types than hire them. I do not think even dukes can hire over 50...
Hang - a long-time member of the Barony who has held every title but Ruler - is able to field somewhere in the neighborhood of 70 SF as a cavalier. I think the most I've ever had Hang field was 50, with a CS of <a lot>.
Cavalier banker dukes who have duchy and realm taxes that funnel money to the banker, and with all lords investing all the time? Hah, not actually going to happen, but in theory, that would allow that one guy to have an awesome unit until he has to repair their equipment damage.
To be honest, all it takes to field a unit of Behexed Artillery is a decent income (more than 100 gold/week) and honor.
So my guess at the reason BoM hasn't taken over AT yet is because the cost of losing even one of those would often offset the loss of about ten moderate standard units. Considering the cost, the smaller numbers that can be fielded by most nobles, as well as the rate of recruit replacement, BoM would probably need at least two such RCs to make an army that might make a huge impact everywhere they go.
More likely that BoM has never had an expansionist mindset, and since the implementation of estates simply has not had enough knights to even consider it. Fortunately, we've always been more interested in taking someone else's gold than their land.
