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 ****, too. To this day, I'm leery of hanging weight way off the back of a bike.
