The objection here is more structural than technical. The famous dropbox objection is 'anyone could do this' - even though they might not have the wherewithal to do so. The objection here is that the open source project is relying on a closed source paid service to do all the heavy lifting.
Someone is going to need to foot the bill, which means this project will eventually have to answer some tough questions about funding, and what the project actually delivers.
It's usually all in the details and delivery (and ya'know we're lazy and lack time to setup stuff locally)
Though I wouldn't really knock anything free and open source either way.