Here Sailfish will be using Mer, but will pose as a metadistro. Since Mer is already a metadistro for vendors - that's confusing. Unless Sailfish is intended as a higher level metadistro, with additional components which Mer lacks, which it intends to standardize (not sure what those are). If those will be open - it'll be OK I guess. If they'll be closed but still posed as meta components - that's very much not OK.

Anyway, it's hard to understand anything until there will be more details. I just expressed obvious concerns which come to mind.

Mer is not set up as a "walled garden". Any vendor can use it (like PlasmaActive already does). My concerns are not about Mer, but about Sailfish.