> Right — but there’s a broad question of if Congress can establish special courts. This is being asked about more than labor, including taxes and regulations.
The issue isn't with Congress establishing special courts – it is establishing these special courts as part of the executive branch instead of as part of the judicial branch.
If Congress was to establish a specialist Article III "United States Labor Relations Court", the constitutional argument being made here by Amazon/SpaceX/etc would not apply. Instead, Congress decided to establish a "National Labor Relations Board", which is not an Article III court, it is an Article I tribunal, [0] and the constitutional question being raised here is about the constitutionality of Article I tribunals.
(I doubt SCOTUS is going to find all Article I tribunals unconstitutional – military courts-martial are an example of an Article I tribunal which SCOTUS is highly unlikely to find to be unconstitutional – but it is plausible they may soon [1] rule against their use in regulatory matters such as labor or securities regulation.)
[0] https://en.wikipedia.org/wiki/Federal_tribunals_in_the_Unite...
[1] https://en.wikipedia.org/wiki/SEC_v._Jarkesy