Nice review, you scored a crazy good price, envious!
I was using a hartmann for decades that was finally falling apart, then switched to a Costco spinner. Made the mistake at stopping in at a high end luggage store during Christmas season and got myself in trouble after 30' of checking out several brands also walked out with a BR. A slight different model U121CXSPW-4.
Like the OP, needed a two carry-on solution that could take me internationally every other week and in a pinch two weeks ( including a one suit/sport jacket ). With Global Entry and APAC, I'm in and out of customs and out of the airport likely before any of the Global Services get their checked luggage
After 6 months of use, no question this thing is built well. Everything the expanding handle, wheels, zippers, pockets everything built rock solid. I've lugged it thru airports, up and down stairs/escalators, dragged over cobbled sidewalks and still like new after 6 months.
The expanding feature then ability to push flat is rather clever, but not clear if that is better than my old hartmann that had zippered expansion where I'd compress and zip, both seem to work equally well.
As to downside of spinners, if you are on trains/subways them wheels are pretty good, the bag can easily run away. I can't remember once where my wheeled Hartman would do that.
I agree with the other poster, I've watched my BR and backpack combo fall over. Physics that can't be avoided with anything heavy on top of it. But a credit to the build of the BR, I've actually picked them both full packed by the top handle almost everytime flying into Shanghai where they never seem to have the escalators at immigration going and we all have to descend the stairs.
Another nit, the design cost of having a perfect lie flat bottom comes with the expensive compromise that the bars are outside and you lose some significant storage space for some rolled items from other designs. They have a cute compartment with zipper, but really what can you put there? No two ways around it, but in hindsight that is worth at least few pairs of underwear and socks. My hartmann which lasted for decades had that internal, but with the trade off of the bumps at the bottom.