I've sold three cars to WBAC - the most recent around 2020 - at the time I felt the prices were pretty decent and because my cars were generally clean there was either minimal or no money knocked off by WBAC when they physically assessed the cars. I also used WBAC offers to get dealerships to up their part-ex offers on a couple of occasions (ie to match the WBAC price). I was surprised on one occasion when a dealer wouldn't match the WBAC price so I sold it to WBAC and paid cash for my next car - (and I didn't buy from the dealer who wouldn't match the quote).
But when I tried to sell my wife's car 14 months ago, a WBAC offer was not good so I got a Motorway quote as well - it was over £2K higher and when I actually followed the process through and got offers from dealers, I accepted an offer around £3K higher than the WBAC quote.