It's perfectly reasonable for, say, a tattoo artist not to be liable for the medical bills, if the ink causes a hitherto unknown allergy to kick in.

Why would it be rasonable? Did the tatoo artist do what is (keyword:) reasonable on their end to ensure that doesn't happen? Did they make information about tatoo ink allergies known to their customers? Do they advise their customers about the allergies? Do they use FDA approved tatoo inks?

It's not reasonable to argue that a streaming service agreement covers liability for being cut in half by a train.

Did the streaming service clearly for example cause magnetic interference and was ruled as a large contributor to the disaster? If yes, then it's reasonable.

Whatever scenatio you think of, there's always room for liability. Some, nay, mlst of it's far-fetched, but not impossible.

However there's at least one thing that's never reasonable, and that's arbitration itself. Arbitration is someone making a decition which can't be amended after it's made. It can't be appealed. New evidence coming to light after-the-fact means nothing. Arvbitration is absolute.

Arbitration doesn't allow complaint. The judgement is final.

Which is fucking ridiculous.

Let's return to your two claims of unreasonability:

It's perfectly reasonable for, say, a tattoo artist not to be liable for the medical bills, if the ink causes a hitherto unknown allergy to kick in.

It's not reasonable to argue that a streaming service agreement covers liability for being cut in half by a train.

There's nothing stopping a normal court from fairly making a judgement. It can be appealed, which is fine.

What isn't fine is giving a company, or a like-minded court sole and absolute jurisdictions over suits against a company by its users. And above that, making said judgements unappealable.

To paraphrase you: there has to be a reasonable understanding of the underlying facts of the case covered. Some claims are clearly ubsubstantiated. Some, however, are clearly substantiated and if the service provider knows and understands that, they would accept the jurisdiction of the court system without carveouts grossly in their favour.