I’ve had roof mounted carriers for around three years. Done a lot of UK driving and trips to and from Les Gets/Morzine. The fuel economy thing is an issue but if there’s you and a few mates going out generally we split the fuel costs anyway. My Oct Est does around 47-49 mpg, with 3 bikes on the roof with people and kit it does 39-40 mpg so not the end of the world.
Getting back from a ride and putting the bikes straight on the roof and out of ther way is a good feature too, yes the car sometimes gets a bit dirty but even I (and anyone who knows me will maybe be shocked by this) don’t mind this.
Security is an issue, but to be honest I don’t feel safe leaving my bike anywhere, so I don’t. Someone stays with the car if we go anywhere or its ‘in sight’.
There is always the ‘hit low things issue’. But i’ve seen enough people reverse into things/cars/people with toe bar mounted racks to realise that it just takes some considered driving to avoid this.
They all have there plus and minus points, but i’m pleased with what i got.
Hope this helps