It just hangs. I can try to leave it for longer to see if it ever launches, but the 25% of the time it does run successfully, it just loads pretty much instantly.
I was able to repro the hang that you're seeing as well as the occasional run. So I tried to update to 440, but the latest I could see was 430. I updated to that and then Steam decided that it needed a specific version of libgl. So then I tried upgrading Steam from apt-get and now it kind of looks like it's trying to build Steam from source...I'll keep trying.