Creating new siteroots in a folder (pid NOT 0) generates wrong and not fixable slugs
Have a sysfolder, containing siteroots. Add a new page in there. The SlugHelper generates a slug different from "/" (because it's neither on pid=0 nor an actual Site yet), but due to PseudoSiteTcaDisplayCondition this slug is not editable (it returns false when not being on pid=0 for whatever reason).
All subpages of this page will have this wrong slug prepended.
You need to create the page first before creating the site yaml because you need the uid for the rootPageId.
In a several-hundreds-domain setup, organised in logical folders, where siteroots are added on a daily base (also by non-developer backend-admins, the devs only add all the domain config only once the page tree "is ready") - imagine what a mess this creates which is only fixable with raw database Access by hand.
(regression, because in v8 one could create siteroot pages without problems)