For the fan, the risk is minimal. Downloading from Archive.org is generally safer than torrent sites because the files are scanned for viruses and hosted on a .org domain. However, users should use common sense: support official re-releases if Cartoon Network ever launches a proper 4K remaster.
However, the ethics of this archiving are complex. If a piece of media is not available for purchase in a complete format, and is not available on standard streaming subscriptions, it enters what archivists call the "desert of abandonment." In this view, the Archive.org upload serves a public good—preserving the work for future generations of animators, historians, and fans—without significantly cannibalizing the revenue of the rights holders, provided they are not currently monetizing the series heavily. ben 10 ultimate alien archiveorg