Bubblewrap doesn't help with protecting DBus bus (and you can do a lot with unrestricted access to DBus), accessing the GPU, Wayland and audio daemon. It cannot protect /proc and /sys filesystems. So it works only for simplest CLI programs.

As I remember, some Node.js utility (maybe npm) had a bug where it hang if there were no /proc filesystem because it couldn't figure out how many CPUs are available. Telegram crashes because of Apple's Grand Central Dispatch library if run in a sandbox without /proc. That's how bad things are.

Also, Chrome and Electron apps use SUID binaries to build their own sandbox. How would you sandbox an Electron app (obviously running an untrusted SUID binary is not an option)?