YouTube has audio in Opus format@~150kbit/s. Opus is a much better format than MP3. Almost all audio is completely transparent at that bitrate, where with MP3s, there are cases where audio is not transparent without using non standard >320kbit/s bitrates (a lot of content is transparent @320kbits/s though).

Now, sites/tools like the one you mentioned take the Opus (or AAC) file/stream from YouTube, and lossily re-encodes it again, probably to a file that is larger than the original, with at best the same quality, but probably worse quality. You obviously can't get better output than the input in lossy compression.

So, the disk space argument is weird if you can play Opus/AAC (should be playable on every device nowadays).

This is the valid part for why you shouldn't use YT-to-MP3 converters.

But there are also invalid reasons why people will tell you it's shit:

• They think all MP3s sound like the shit ones from a decade (or two, or three) ago, using low bitrates and/or created with shit encoders. In reality, not all MP3s sound like shit, but vigilance is needed at every encoding step, as is the case with all lossy conversions.
• They are conflating the quality of the conversion, with the quality of the source, and think the bad quality of some user-uploaded YouTube content is due to the lossy conversion done by YouTube, and/or the MP3 converter re-encoding from YouTube. Content uploaded by the copyright holders (assuming basic competence) does not have that problem at all.