Eh, all the providers have done that shit once they account for the real cost of storage and realize the demand is outpacing their acceptable losses on "free" tier products. DropBox will be next once they start repaying the loans for that shiny new datacenter infrastructure they are building to replace AWS.
As for the OP's question: The security, relatively speaking, is as good as any of the other public cloud storage providers. Probably better actually. Microsoft has put some serious coin and design chops into these products to compete with AWS and Google.
I wouldn't worry about it any more than you worry about the security of your email while it's on the remote server or any other Internet bounded service. OneDrive is a good product but to be in full disclosure, I've never used the free version.
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Use Google Drive or Dropbox instead. Microsoft recently fucked over people with free OneDrive accounts by dropping the storage limit from 15GB to 5GB. They also stuck a 1TB cap on people who paid for "unlimited" storage.
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