Can you trust anyone who claims that things are end-to-end encrypted? Microsoft could take the point of view that you are one "end" and DropBox is the other "end". If they encrypt the data in transit and decrypt it on their end, it's still technically "end-to-end encrypted".

They could also just lie. Having a company claim end-to-end encryption still means that I have to trust that the company isn't being sleazy.

The only encryption you can really have some measure of trust in is the encryption you apply yourself.