I think Meadow and Pi have two different purposes. Take something simple like power consumption. The STM32 can be put into a low power mode and can run on solar power or batteries a lot more effectively than the Pi. The Pi normally runs on about 300mA which will put a large strain on a remote system running on batteries.
we plan to make meadow embeddable on your own circuits, so at some point we will open source it, as well as provide guidance and such.
The above comment on the Kickstarter pages shows the commitment to allow Meadow to be embedded in products. The only way to embed a Pi into a product is to use something like the Pi Zero (for a small footprint). This will add to the overall size of any products making small products unlikely with the Pi.
Meadow also allows reuse of skills from the desktop and web world. Being able to use .NET in Azure to collate sensor data as well as on IoT devices lowers the cost of development.
As with all things, it is a case of making a choice that is right for the product under development, something I am sure you are fully aware of.