I use the Bentley brush for everything, and it has two 2ml pencil jets, it's fine, a perfectly good brush and lasts about 6 months or so.
It is slower on first cleans, low density of bristles means that on dirty glass you have to scrub very thoroughly, but how many first cleans do you do a week?
As the bristles are also very springy, it is only the bristle tips that are in contact with the glass, with a brush like to Ionics double trim, there is a greater density of bristles per square inch, and they are floppier too, hence it is not just the tips that scrub the glass, ergo it will wash any given area more thoroughly. But of course it is also way heavier and more likely to retain contaminants.
On big panes of glass it is fine, I do an awful lot of commercial stuff, loads of shops and offices with huge panes of glass, absolutely no problems whatsoever, if you feel it is slow, just increase the flowrate.
You can DIY them of course, but at under £20 why bother? Fine though if you enjoy dabbling then knock yourself out!
Most of the brushes in use are not specialist window cleaning brushes, they are mostly floor brushes that have been adapted, or car washing brushes in the case of many of the Vikan ones.
No one brush is all singing, all dancing, and it is so subjective, what one person loves, another will hate.
Give the bentley a pop, DIY it, blow a fiver! What have you got to lose?
Ian