It should be noted however that this is somewhat out weighed by the fact they carry far more people than 'regular buses', so the real cost benefits for an operator are in there being less staff required to provide the same "capacity per hour" on a service. In the same logic a tiny Solo is going to be cheaper to run than a full size B8RLE.
In the case of Aberdeen the Artics are typically allocated to a small group of services, however if required they can and have operated on all bar two routes. In terms of street furniture, maybe it's down to the different vehicle types however generally here that's not been an issue either give or take a few specific locations, in fact in a few cases some of the tighter turns in the city are easier in an artic as they have a tighter turning radius than a single decker.
Mercedes still offer (and build) the
O295G Citaro, if a big enough order was placed to justify starting a production line i'm sure ADL would look in to it. Iirc Irizar and Caetano both have Artic products that are offered to the UK Market (albeit none built or certified as of yet).