Maybe other people's experience is different, but for me, when you do that first giant world update of a Sabayon-Linux installation, you'll encounter a few 'problems' even if things go relatively smoothly.
You have some potential for adding additional problems by interrupting the world update for a reboot, but I've done it.
Like you say, all of us aren't in a position to guarantee a computer's availability for a multi-day update. If you are willing to accept some risk in generating a problem that you'll have to fix, then go for it.
A couple of things I've learned to watch for:
(1) Try to break the update in the middle of a compile cycle. Try NOT
to interrupt the update when it is merging the package at the end of installing a package. In theory, sandboxing will minimize that time window when you can really screw up installing a package. But there is a point at the end of the install that can really hurt things if it is interrupted.
(2) If you use some of the tricks for continuing the update by skipping the broken packages (such as: 'emerge -e world || until emerge --resume --skipfirst; do emerge --resume --skipfirst; done'), you have to be careful how you interrupt the update. Doing a simple Ctrl-C won't kill the update when it is called in this manner. It just kills the update it is currently working on, and goes on to the next update. I'm not 100% sure how to stop an update once it is started with this syntax. Sometimes, I just had to cross my fingers and re-boot.