Is this the new Unger sill brush? Can't comment on that one. The old ones were ludicrously heavy.
I've only ever used a rectangular Vikan monofilament once, about 6 years a go, I didn't even finish the window with it. I don't care if they are good or not, they old ones were definitely not supposed to be used for window cleaning, way to heavy. From specs, the new ones still seem way to heavy. Worked for a while with an Ionics dual trim, but since the first Superlite brush came out about 5 years a go, I've had nothing but Gardiner brushes on my poles. They last for anything between 1,5 year to years, but even if they lasted a month I'd still use them.
Carbon gooseneck and Xtreme brush and you are flying.