They're considered generally enforceable.

https://corpgov.law.harvard.edu/2018/06/08/the-enforceability-of-employment-arbitration-agreements/

On May 21, 2018, the United States Supreme Court, in a long-awaited decision, held that employment arbitration agreements with class action waivers requiring individual arbitration are enforceable under the Federal Arbitration Act (the “FAA”), notwithstanding Section 7 of the National Labor Relations Act (the “NLRA”), which protects employees’ rights to engage in concerted activities.

But they might not be. Sure, if I had some case related to this I would get legal advice.

https://www.lexisnexis.com/community/insights/legal/practical-guidance-journal/b/pa/posts/drafting-enforceable-arbitration-agreements-hottest-issue-in-contract-law

Arbitration agreements are supposed to ensure that disputes are resolved outside of court, and that’s why it’s an irony of almost cosmic proportions that “the enforceability of arbitration agreements is likely ‘the single most litigated contractual issue’ today

https://www.natlawreview.com/article/ties-bind-you-arbitration-agreement-enforceable-and-binding

But are they enforceable?

The answer, fortunately, is yes, but it is important to keep in mind the most basic characteristic of arbitration agreements - they are contracts. Both Federal and State laws foster a strong policy favoring arbitration, but each provides that the enforceability of agreements requiring arbitration for work-related disputes will be determined by applicable state law regarding contract principles.
The majority of published case law suggests that arbitration agreements are most commonly challenged on the basis that they lack mutuality of contract, lack adequate consideration for the contract, or are unconscionable. These tripping blocks can be avoided by drafting a carefully-worded arbitration agreement and providing employees ample notice and opportunity to review the agreement.

The latter is why 23andMe is giving people the chance to review the new TOS first. It also is probably different for a TOS vs an employment contract.