bootstrap-vz might not be the magic bullet for everyone who wants to get an image for virtualization going.However, I do think that finding a bug hidden between ~3.700 lines of code (bootstrap-vz) is a bit easier than doing that when looking at 40.000 lines of code (packer).
If I understand the inner workings correctly, packer does not run things as a chrooted environment, instead it boots things up and then connects via ssh after everything is installed (not sure how the installation works, probably preseeding).
I'm impressed that Mitchell has managed to maintain it and that everything works across providers AND distributions. But dayum! That's a lot of code for bootstrapping.