I used my FlexiroamX sim sticker in Japan earlier this year without issue. It took a while initially to find the network, but then I had good data coverage for the week that I was there. The downside is that you cannot receive calls or sms to your ‘home’ sim while the flexi is running.
As for paranoia about your phone getting stolen, I wholeheartedly agree with the previous posters who say that this is not an issue in Japan.