I dont have parallel boot enabled as well as it is disabled by default. Looking at this Systemd post triggered this thought to experiment with it one weekend. I have this laptop where I run Gentoo like a control freak. There is nothing in it other than the core system running as openrc services. So, I might play around adding readiness gates to those and see if anything at all can be parallelized.
I've been running OpenRC in parallel boot mode on my Gentoo systems for ages now. While I'd never bet that there are zero bugs in it, from what I've seen it works fine as long as your services correctly specify their dependencies and -if needed- start ordering.