have always wanted to know, how watertight are those clauses? surely legal recourse is some sort of right that'd trump a lot of these type of boiler plate contracts?

Courts have consistently held up Amazon's arbitration clause for sellers.

US courts treat commercial litigation and consumer/personal differently. Commercial is "letter of the contract" (you should have talked to your lawyer before signing), whereas consumer has to be reasonable to the non-expert public in the long run, and is also subject to tons of social justice goals.

US courts have upheld arbitration clauses in shrinkwrap consumer contracts.