I mean, that’s completely subjective since you just wouldn’t be able to download those parts from another seeder. You may have had issues and just never noticed because it shows up as not having any valid seeders. There’s a reason every torrent client has an option to verify your data, though.

You're paying them money, so it's in their best interest to keep hosting.

The uploader uploads their stuff to their own Usenet provider (whom they're probably paying for). Usenet servers are frequently mirroring/syncing with each other. So very quickly after the uploader uploads, you will find their post on your Usenet provider, and you download directly from them.

If a Usenet provider someday decided not to host any more, they would be out of business (because who would use them), and so you'd switch to a different Usenet provider, where you'd find exactly the same stuff mirrored.

Usenet providers compete/distinguish themselves mostly based on:

• Cost (duh)
• Speed (duh)
• Retention. This means "how long is a post kept on our servers after it's been uploaded". Some cheaper providers might have only 30 day retention while some might have 180 day retention, etc. If you're only interested in recent posts/releases, it might not matter as much to you.
• Tooling. Most Usenet providers have a web-based interface, with varying levels of service. Can you search for a specific filename, do different types of filtering, etc. Many providers will automatically package together files that have been split up, so you only have one download, and don't have to worry about par files and unrar and all that. Some will give you thumbnail previews, or even short video previews, of videos before you download, so you can check quality and language (important!! Some people on Usenet don't even bother to label the fact that they're uploading, say, a Spanish language version of something)
• Obscure communities. Many people do still use Usenet for discussion, its original purpose. If that's you, you're going to want to check that the provider you choose is going to have alt.fan.obscure.howdy-doody-berenstain-bears-crossover-fanfic.bonk.bonk.bonk or whatever weird interest you and 3 other people in the world have. You might think since the discussion communities are so low-bandwidth every provider would just carry everything, but you might be surprised.

A Usenet server will typically host the files and then delete them after a set period of time. Most servers won’t typically go less than 30 days and a few will go for months. It’s what makes the OP question more of a decision between wanting something as soon as possible with quick downloads vs wanting it any time outside of that initial period at whatever speed it’s available at.

It’s just a historical artifact. Most scene groups release to Usenet first and then, from there, it gets to torrent sites. If you’re also on Usenet, you can just grab the files right away and then start seeding a torrent immediately.