I have yet to use entrenchments but here are my observations and suggestions any way
I think entrenchments should be an evolution of breastworks. That is, first you would have to build breastworks then you would have to convert them into entrenchments. The factors for each should reflect that entrenchments are really much improved breastworks.
It would really be nice if both were treated as a progression rather than there or not there. That is, breastwork-1, breastwork-2, breastwork-3, entrenchment-1, entrenchment-2, entrenchment-3, etc.
I would also like to see it change to occurring for all facings of the unit at the same time. I don't like having to expose flanks in order to complete a breastwork. The progression idea would allow this to work.
I would also like to see terrain modify the build rate. Woods should help breastworks, hinder entrenchments. Rocky terrain would have a similar effect. It is easy to pile up rocks to make a breastwork. Hard as h*** to dig a trench through it.