Sorry to revive an old thread, but I just logged back on after a couple years away and wanted to type some of my thoughts about UE in China:
You're generally right that UE can be very dangerous in China, especially when it comes to (old) military installations or areas of "national interest".
But like in Japan, under all 'normal' UE circumstances, you can usually just play the dumb foreigner card and get away. I've been caught on rooftops, beyond fences, construction sites, a rubber plant, etc. Every time I just pretend I don't speak Chinese and walk out (or usually get escorted out). In general, the security / police don't want to deal with the paperwork.
Another point: many of the UE locations in China are fairly unsecured. There is a culture here in China that I call the 'cha-bu-duo ethic', which basically means half-assed, jerry-rigged, ignored, or otherwise shoddily done. Almost every rooftop is either A) open, or
unlocked in China. I have never had to pick a rooftop door lock.
I can't speak to whether or not Japan has better sites, but China is also ancient and filled with old places -- perhaps more so than Japan. Lots of old and abandoned sites, even full districts of cities, were 'old' only a few years ago, and have since been abandoned in favor of the 'new'. I'll make another post some time of the abandoned house we found in Chongqing last year. China only started modernizing in the 90s, so there are troves of old brutalist buildings built back during Mao's time that are mostly abandoned, now that people have money. In Nanchang, I found a mostly abandoned building, and inside one of the apartments it was like a time capsule from the late 90s / early 2000s, with bold and bright 90s Hong Kong singer posters, old children's toys, dirty floral duvet, old school wood chairs, cassettes, CDs, etc. And as a prime example of the UE sites in China, of course there's the underground city in Beijing (although I've never managed to find an entrance). So I think there's a plethora of old and decaying places yet to find in China, and many are REAL af.
Hope you will consider.