Unetbootin as shipped in Tumbleweed breaks opensuse isos. Files get mangled on copy to stick. You can try with current installer isos and then compare /repodata from the iso with the /repodata on stick. The stick is bootable but installer will complain about missing files. Syncing repodata from iso over to stick makes the install work. Trying again with Tumbleweed Rescue iso and again missing files.

As unetbootin seems to be dead upstream, maybe just drop it from the distribution. Don't think this is worth investigating unless someone immediately recognizes the problem and has a fix.