Using the casper/squashfs method comes with long USB device initial time at boot, although it has wide range of spaces. The uncompress time is a bit slow too.
But packed all things into initramfs results in bad loading time, and it's not quite functional.
However, I would like to try a hybrid method in the next release of xPUD, and I just draw a diagram:
Hopefully this can get the advantage on both side, and I'll make some experiments of pivot_root / switch_root this week. :-)