Here is my "non-scientific" feedback, as I had not deal with WeatherDuino myself, and some of your inquiries, were mine as well.
Besides some similarities (as I learned), dealing with RPi is not the same as dealing w Arduino
- RPi is to behave as a "micro-computer", while Arduino is to behave as a device.
- RPi you may work with desktop, programs and packages, while Arduino may require more programing skills
- RPi you connect the usual computer periphericals, while Arduino might require a lot more assembly and welding stuff
On my side, while I truly enjoy to play with RPi, with Arduino was not the same pleasant experience. Although I'm a truly handyman, to deal with tiny welding was frustrating (just for a start, not even mentioning programing).
WeatherDuino seems to come mostly assemble, but to note that you have to deal with both transmitter/receiver and still deal with all sensors (and as you note, compatibility may vary).
Bottom line, WeatherDuino might be a interesting project, but think only if you already are well rounded with Arduino (not RPi), programing, tiny welding and devote time for the experience... as a hobby.
Otherwise (as I may have realized myself), may be more "trouble" than "fun".
Wishing well for those going through this assembly. Wishing well to the WeatherDuino project. But I may not go through this path...
Best,
Xant