SpiderOak has been saying they "expect to make the SpiderOak client code open source in the not-distant future" [1] for years now, and their code still is not fully open-source. I completely understand their situation and know it's not always possible to fully open up the code, but I'm not able to overlook this deal breaker either.
Encrypt locally using only open source tools, then sync with whichever online backup/sync provider you like. It's the storage version of a 'dumb pipe', maybe we call it 'dumb containers'. Everything else is marketing bullshit which will evaporate once that first official demand for your data arrives.
[1]https://spideroak.com/faq/questions/35/why_isnt_spideroak_op...