At the present time, iScreensaver Designer 5.2 provides support for macOS 10.13 High Sierra, but there are some functional limitations.
Please read the 10.13 High Sierra Tech Note
as well as the Release Notes
Due to changes made by Apple, iScreensaver Designer version 5.2 does not currently support macOS 10.14 Mojave.
Please read our forum post
for the latest updates.
Status of Mojave investigation?
How's the investigation into Mojave going? It seems it's broken lots of screensavers not made with iScreensaver, but I'm also having issues with screensaver made with iScreensaver 5, particularly the ones based on video files; the Flash-based screensavers seem to work OK.