I have the one supplied by JDTandems which works well but I did not check how far it has to stick out at the rear in relation to the tailgate opening. In fairness to JDT I have changed the car since they supplied it. The front fixing cannot be moved and has to attach to the front roof bar so this limits how far forward you can fit the tandem carrier. I have had to limit how far the tailgate opens so that it does not touch the tandem carrier. That stops it from damaging the paint on the tailgate. Instead I bang my head on the tailgate!

I find lifting the rear of the tandem onto the carrier a bit of an effort but being over 80 doesn't help. It's easy when the car is parked on firm level ground but in the real world you find you are standing in a dip so the lift is a few inches more.

We also have the roof carrier from JDT and find it is excellent although agree that you need to check the tailgate - we have a saloon car so this is not an issue for us . Regards moving the bike up onto the roof we have a small folding stool .

I have used the Helton Tandem carrier for many years and been very happy with it.  The standard model is designed for square section bars but the maker will adapt for different bars at small cost.  It is very adjustable and easy to load up.  I feel more comfortable with the tandem flat on the car rather than vertical.  But check the distance between your roof bars as I needed to have the standard model carrier extended to fit my Mondeo.  Tailgate impact can be a problem - but you either have to be careful or buy a longer car!

There was some considerable discussion about these last year I think(?).  The lack of any 'kite' mark or formal validation against safety standards was raised.  My view is that anyone who services their own bike is competent to perform a general safety and security check on these carriers and should plan to do so before every use.  The roof bars are tightly controlled for safety but users must then be responsible for what and how they mount any loads.

We've got one of the cheap ones available for about £100, which has been fine. It wobbles a bit more than I really like so on long journeys I just put an extra strap to an adjacent upright and it's fine.

As above, we have the JD Tandem one on Thule bars and it's excellent. Very stable. Driven round the country with the tandem on that one. Hard work getting the tandem up there but you quickly get into a process which limits effort.

We have one from JTD tandems and have been very happy. I am 5" 9 and don't have a problem loading my wife is 5"2 and struggles a bit fastening the down tube but we have found if you park next to the kerb you are a few inches taller and it helps 

We, being vertically challenged (!), carry a collapsible step stool in the back of the Yeti (a tall beast!)