The rack isn't the weight limiting factor, the subframe is. There have been cracked subframes from loading up the rear of the bike too much.
As a benchmark, I use a piece of 1/4" aluminum plate measuring 12" x 18" mounted on the top of a Givi (welded steel) rack. I put 40 to 50 lbs. of camping gear on it 3 or 4 times a year and have taken off for 1000 mile + camping trips with absolutely No Problems with the handling or subframe. I don't ride hard (mostly interstate) with all that gear strapped on, but I make it there and home with no problems. When packing, one thing that's really heavy is water. Better to wear that on your back or mount a canister on your passenger footpeg like a Skyway or other such hydration system.
I ran my large bag transversely across the pillion and used it as a backrest. Worked very well and then Garauld's (different type) backrest and rack had a much smaller bag with light but quickly-needed stuff like different gloves, first-aid kit, etc. Over the course of 6500 miles I got to packing and unpacking quickly. I liked the large bag for the long trip. But I also quickly realized there were things I didn't need for the trip, that won't be coming along next time.
Dang, that reminds me of when I used 4 bungee cords to attach a 25-inch CRT TV to the luggage rack of a Yamaha XS-650 back in the early 80's. I only needed to get from the east side of Las Vegas over to the west side, not more than about 8-9 miles back then, but every tiny bump felt like it was launching the front end. Side to side stability was shit, too. To this day, I'm leery of hanging weight way off the back of a bike.
