Whats so wrong with the Preston Bus website ? I would expect Transdev will want some customisation to, for instance, not have the 36 buried down a list of services.
The harder to get maps. Whether that be network map or ticket zone map. The tickets are tends to get very cluttered. Timetable pages you just get a random break in the timetable and then you scroll down to get the rest of the timetable.
This is how the website can look on mobile, this example was taken from WhiteBus. (Photo shows how the layout isn't user friendly. Download fares link hidden behind other text. Select mobile ticket isn't shown very well.
Depending on how well the site is maintained, you could end up with a site like Adventure Travel where routes are duplicated many times and you can't find any information. Or Diamonds site whereby you have to keep selecting the area and/or there is a shed loads of area specific information in general areas as everything is one one website.
Plus and down side is for Passenger, if people select a date, they can see the timetable for that day so helpful around Christmas time. No provision on RDM sites for that I believe. Only 2 up sides of RDM over passengers systems are the ticket list can be reduced a lot as RDM groups tickets rather than Passenger where every one is listed separately. The other benefit is passengers aren't forced into selecting a date for the timetable or download the PDF, the basic Mon-Fri timetable is all there.
Suppose there are up and down sides to both but on the whole, I've always found RDM a much worse platform than Passenger.