Although, I do have high hopes for Key:Value store data repositories.
TANSTAAFL.
Remember only successful companies have to worry about scaling. Un(/not yet)-successful company have to worry about shipping first, then scaling. So perhaps Saucelabs reached that milestone, and they simply have outgrown the original technology that helped them ship.
Who knows, maybe if they had spent time re-implementing a REST interface on top of MySQL or re-implementing Futon for doing debugging, we might not even have heard about Saucelabs these days.
It did make a few things harder later on but we were better equipped to make a change around the time we actually needed to. If we had spent a lot of time up front developing for SQLite, we'd have paid an overhead tax for years for no good reason. And possibly wouldn't have made it to the point in which we needed to make a change.
That said - our use case was incredibly narrow (read only data store, server outputs JSON anyway, etc.) and so we made a reasoned choice.
If we needed complex queries/joins, updates/deletes, incremental loading, etc. - then a JSON store would've been terrible.