Are bicycle rear racks and panniers suitable for cross-country riding?
I'm a member of Warm Showers, and regularly host cross-country bike tourists and get to check out their gear. Except for the occasional monk with orange robes and a small backpack, what bike tourists choose has been surprisingly uniform. 

It's usefully Ortlieb panniers on front and rear racks, and sometimes a handlebar bag. The details and presence of the handlebar bags vary some, but the choice and use of Ortlieb panniers has been a near constant. I don't own any yet, but I suspect they are chosen because they are waterproof and durable-- two key qualities for a long distance trip!

Here are a couple sample photos of bike tourists I've hosted (both with Ortlieb panniers)





I would add: don't worry about the weight-- with a proper rack, bike tourists sometimes carry 50 to 80 pounds of gear, much of it on the rear wheel.