IMHO video links should always be YouTube first, then third party mirror second, in the body or comments. Here are the reasons:

1. YouTube is the primary source. Always prioritize the primary source when you're sourcing.
2. Think of the URL being shared as the primary key to a database record. Lemmy can use it to detect dupes. With YouTube URL there's like 3 forms, discounting timestamps. With piped links the possibility expanded to 3+(number of piped instances on the web). This massively increases the likelihood of accidental reposts.
3. Give people a choice of whether they want to watch on the original site, or on a piped instance of their own choosing. When you link to a piped instance link you rob people of that choice.
4. A piped instance can choke or go offline at any time. YouTube is close to 100% uptime.
5. Userscript wise, it's trivial to map YouTube URLs to a user's piped instance of choice, but it's so much more tedious to detect and map every single piped instance.