We carry our recumbent trike in the back of a Citroen BX hatchback. The front protrudes through between the front seats, and we wrap it in padding for safety. At some point I may hinge the front boom so it will fit in easier. This will also make it easier building the cardboard box for transport.
We have also carried it on the roof, with and without roof racks, but the drag from the seat is high. For long distances I would suggest unlacing the seat. Exposure to rain and bugs etc is also a worry.
Trailers are another option, but unless you add some weight, they can be very rough on the trike.
A tow bar mounted rack or platform is another option, but would need to be properly engineered as the weight is a long way back.
As a trike sits flat on 3 wheels, it is generally easier to transport than a conventional large wheel bike.
As far as riding them, they are faster, safer and much more comfortable than a conventional bike. You do need to adapt and I would not suggest going on a GVBR without doing some training beforehand. Hills can be a little slower, but downhill and flats more than make up for this. With the right gears you also never need to walk as they are stable no matter how slow you go uphill. Long days with strong headwinds are a breeze. Strong cross winds are not a problem. Cars give you far more room. They are also easier to park, without the need to find a post, at stops and in camp.
While there is no doubt trikes are better than conventional bikes for rides like the GVBR, there are some negative aspects. In heavy rain you get very wet as you are down in the spray from car tyres. On hot days you are down near the hot bitumen and lying flat. Long sleeves and pants are needed for the sun.
For the NSW rides with 1000 cyclists, the slower uphill faster downhill characteristic has not been a problem. However, last year in the GVBR it was an issue. With so many cyclists on the road we did have a number of incidents where cyclists did not keep a good lookout behind and would wander out in front of a rapidly overtaking trike. While school riders were the most common offender they were not the only ones, and all riders need to keep a lookout for overtaking riders (trikes and bikes).