For example some common .deb applications that have a demonized component ship systemd service files, so you have to figure out their replacement. Again just a little work, and not a big deal. It's normal in something like Slackware, but the bsd-style init is really simple there.
I had to do the firmware dance initially, which Debian sorted out a little while ago, but that isn't a big deal. The experience did make me realize how widespread the systemdification of things has gone, which of course is one of the goals of the Devuan project - to avoid this.