I have had over 60 people stay at my place with a variety of bikes and I started to look at their reliability. Here is my list of bikes and some of their problems. I will try to be as objective as possible.

2 BMW R100. 1 had problems with a small spring in the gear box which was fixed very quickly. The other had a stud strip in the cylinder which was also replaced easily.
5 BMW 1150 GS. 3 had final drive problems
5 BMW 1200 GS at least 3 had final drive problems
12 650GS most had no probllems but some had rear shock problems and water pumps
2 Aprilia 650 pegasos 1 had a rear shock problem
1 Buell Uly It hade no problems
3 Honda Transalps 1 had no serious problems. The other 2 are somewhere in South America and have had some minor problems}
1 AfricaTwin. As far as I know no problems
1 Kawasaki 750 Sceptre from the 80's . No problems
8 Kawasaki 650 KLR´s. 1 had it's engine rebuilt 3 times
2 KTM 640. 1 had problems with valves.
4 V-Stroms only minor problems
1 Suzuki GSXR750 with Jesse Panniers- No problems apart from the rider suffering from a sore butt
1 DR650 No serious problems
1 Suzuki ts185 No problems
2 Ural with side cars No serious problems 1 of them is somewhere in Central America
1 Yamaha 125 Lots of problems but lots of miles. It had been from England to South Africa and was a little the worse for wear. Since replaced with a Honda 125
1 Yamaha 500xs from the 70's Lots and lots of problems
1 Yamaha Seca 600 It had several problems but all easily and cheaply fixed
In conclusion I would say that any bike will get you there and back but that all of the Jap bikes (with the exception of the Yamaha 500XS which was in bad condition and I think was a POS in its day) do it relatively easily and cheaply. The 650 BMW seems to be a very good bike although the rear shock and water pump are a problem. The big BMWs are great if you have the money and the warranty. They are great 2 up and the BMW dealer gives you free coffee.
I will take a long trip in the not too distant future and am almost sure I will do it on a Jap bike.
Your experience may vary depending on rider's ability and mileage.
If anyone is interested in a place to stay in Mexico City our door is normally open.