I mostly went with them because of their privacy package. They do simplify the process a little bit by putting a nice UI on top of this more manual process [1]. I'd rather pay a few extra pounds than risk submitting wrong information thus creating more trouble further down the road. Perhaps, FUD was playing a role too.
I recently did it, took about 10 minutes online I think. I would guess the services make things easier for you by suggesting a default number of shares, and automatically filling in director information etc.
My accountant also would have done it, but I wanted the company in existence quickly so I could get contracts signed.
[1] - https://www.gov.uk/limited-company-formation/overview