Soapbox isn't any more mature than Pleroma or Akkoma in terms of software.
It's a hostile fork of Pleroma that regularly merges upstream patches from Pleroma. The lead developer, Alex Gleason, got kicked off of the Pleroma project when he called other contributors names for reverting a patch he made that introduced code smell[0]. Said patch was controversial at the time of introduction, but got bludgeoned into being merged because Gleason was by far the most active contributor. It was removed with consensus when other developers became more active, although from what I can tell, Gleason didn't bother to participate in that discussion.
Soapbox is his "taking the ball and going home" answer.
If you want a non-technical reason to avoid it/consider it immature; Gleason cried "cancel culture" when removed from the Pleroma project, even though it pretty much has nothing to do with it. He's a demagogue who used his getting removed from the project to further advocate for his own political beliefs (which is being a TERF). This in particular means that the overwhelming majority of Soapbox instances tend to be maintained by the people that think that Pleroma is maintained by the people Gleason dislikes[1], to the point where some instances automatically defederate from instances using Soapbox's frontend[2].
[0]: Pleroma uses a RESTful API for routing. Gleason wanted to add a bunch of alternative endpoints for no other reason than "this is what users are familiar with on Twitter and my frontend for Pleroma wants to rely on those". The patches were reverted eventually on the simple reason that they're cruft.
[1]: Which is extremely funny considering an early criticism of the Pleroma project was that due to it being easy to host, it wound up hosting a lot of instances that would fit this bill. Now most of those migrated to using Soapbox.
[2] Funny for the exact same reason as [1].