Already the Nextcloud 12 documentation says
Technically Nextcloud can also generate the previews of other file types such as PDF, SVG or various office documents. Due to security concerns those providers have been disabled by default and are considered unsupported. While those providers are still available, we discourage enabling them, and they are not documented.
I have not found anything more current. Should we still refrain from using it? I would be mostly interested in video thumbnails, office documents not so much.