They are less stable and adding them or replacing the channel ID ones by them should be discouraged. 

https://chatlogs.metabrainz.org/brainzbot/musicbrainz/msg/4596074/

Google makes it pretty clear that they are not stable and can even be reused by others.

It turns out that shortening a custom URL by removing “/c” is contrary to documentation not guaranteed to work in all cases, see MBS-11997

Custom channel URLs also make it harder to programmatically use them, e,g. downstream data users, editing bots, or duplicate detection:

• https://developers.google.com/youtube/v3/docs/channels
• https://developers.google.com/youtube/v3/guides/working_with_channel_ids

Furthermore it seems that channels can have multiple branded URLs:

• edit 70397272 https://www.youtube.com/KingNae
• edit 70330322 https://www.youtube.com/NaeOfficialkingnae

Both of them have the same canonical channel ID UCxLCxSB1v8k3Rk0-HIz0OGg:

<link rel="canonical" href="https://www.youtube.com/channel/UCxLCxSB1v8k3Rk0-HIz0OGg">

A few examples where YouTube links are replaced in either direction:

• edit 69574466
• edit 69535828
• edit 69491242
• edit 69386197
• edit 69305504
• edit 69273909
• edit 69171477 (old custom URL doesn't work as of now)
• edit 69137765
• edit 69023049
• edit 68901823
• edit 68842473
• edit 68554513
• edit 68497369
• edit 68497368
• edit 68456125
• edit 68431601
• edit 68386966
• edit 68313182
• edit 68168182
• edit 68163935
• edit 68105961 (old custom URL doesn't work as of now)
• edit 67984794
• edit 66988675 (I remember the old custom URL didn't work when they removed it but now it does)
• edit 67935147
• edit 67823286
• edit 67620447
• edit 70053601
• edit 69760037
• edit 69753187
• edit 70080516 (admits it's for marketing)
• edit 72516055
• edit 78842472 (URL returned by search)
• edit 79957050 (channel ID was already in use, someone changed to custom one and just a week later it broke)
• edit 94028143 (channel ID was changed to handle, a few months later handle was changed and URL broke)