For a person who is over 25, already has full no claims bonus and a lump sum in the bank, it is significantly cheaper to buy a secondhand vehicle and run that instead
This isn't necessarily true though. Me being me, I did a spreadsheet before deciding to get one. 
I have an excellent credit rating, and my motorbility car did require a significant downpayment (as most do now). I have a full no claims bonus from private ownership before moving onto the scheme, and have always bought second-hand cars previously. With the depreciation, servicing, MOTs, repairs, paying insurance, breakdown cover etc, it works out significantly cheaper for me to have a motorbility car (including using the mobility part of my PIP to "pay" for it) than it did to buy 3-5 year old cars myself and maintain, repair, MOT, insure, service them etc, per year of ownership/ use. But with the bonus that the motorbility car is new so less stress because it's less likely to break down and there's no risk to me of a sudden expense if something goes wrong with it, or the hassle of having to sell the old one when it needs replacing, or worry about buying a new one with something seriously wrong with it, or the insurance premiums rocketing upwards every year as they were doing even though I'd got an even longer driving history with full no claims, etc.
The scheme is decent value. But not such good value as it used to be when most cars didn't also require larger downpayments. That's partly due to supply chain issues post-Covid so may improve in future years. But it's also largely down to GBP having been massively devalued ever since the Brexit referendum and never recovering to its previous levels because of the obvious effect that decision would have on the British economy.
Of course if we got a Government that would remove the unnecessary trade barriers and rejoin the single market and customs union then this might also improve, along with many of the other cost of living pressures and lack of funds for public services and low productivity making all British people poorer year on year, but no politicians if any party seem to be proposing to take this obvious step to stop impoverishing British people so...