OnyX and High Sierra
Most users have no problem with OnyX for High Sierra. However, some encounter issues such as being blocked from reading the parameters, being blocked from verifying the structure of the system files, having OnyX launch once but then being unable to relaunch it successfully (problems encountered sometimes if the content of the disk is APFS encrypted). Run OnyX take a long time if the Console application has been run earlier.
High Sierra has many issues unrelated to OnyX. These include taking a very long time to restart from an Apple File System (APFS) disk, experiencing a black or white screen, having Wi-Fi problems, the Mac being slower than usual, the Startup Disk preferences not displaying the disks, many malfunctions occurring in Mail, and so on.
The disk needs to be repaired but Disk Utility finds no problem
Sometimes, the disk needs to be repaired, but when attempting to repair it, the Disk Utility application finds no problem. It is a Disk Utility bug that does not visibly recognize all errors.
Then, you will need to repair the disk using the fsck command in single-user mode.