These kind of converters look like an ideal and affordable way to extend your personal music library.
Afaik Tidal business model per definition does not allow copying DRM protected contents, just temporary downloading on the device – you will never be owning the content legally.
From the artists’ point of view, their revenues depend on the number of playback, downloading the contents would be violating their interest. Just like downloading contents from newsgroups is illegal in every country.
Please note that the AudFree Terms of Use is explicitly stating “make sure the copyright law in your country allows copying digital media files”. Even in my country, which has been very progressive when it comes to both uploading and downloading of contents, I highly doubt I could successfully win a case like this if Tidal was to prosecute me over downloading ‘their’ files. Even the AudFree terms of use will exclude any claims in that regard (see ‘Indemnification’) and you’ll be on your own.
What is the legal status of AudFree and similar companies, their products and where do they reside? What if Tidal decides to incorporate a new code in their database that would prevent copying content via AudFree? Would you get refunded by those entities? What if Tidal, based on your personal (meta)data decides to claim any royalties from you based on the list of contents downloaded by you that have been freed of DRM protection? Not trying to spook you here but they should be able to do that with just a single mouse click.
Many audio playback (edit:) devices nowadays are ‘Tidal ready’ in order to keep that business model in place for both Tidal and the artist. To be on the safe side and if you choose to stream content (no ownership) rather than downloading it (HD-Tracks and others, you own the content legally) just make sure all your devices are capable of streaming/temporary download the content.