They have a list at
https://www.ventoy.net/en/distrowatch.html of some supported operating systems.
It supports most Linux distros and it also supports FreeBSD.
Some operating systems that it failed to work with according to that page include, among others: ReactOS, SmartOS, Haiku and OpenBSD.
They have another list as well of tested operating systems here: https://ventoy.net/en/isolist.html
According to that page it supports many versions of Windows too, as well as VMware ESXi, Citrix XenServer, and Xen XCP-ng.
So probably a good idea to check those two lists first before choosing which operating systems you want to use with it, if you are looking to get something that works with the least amount of effort.
And if you have time to spare then try some that are currently marked as not working, or which have not been tested, and figure out how to get those working too and contribute that info to the Ventoy maintainer via their GitHub :)