If we define safety as "zero risk of data loss", cloud storage is not wholly safe (some companies have gone bust, others have had technical problems), but then nor is any individual storage device. Google accept no liability for anything that happens to your data beyond whatever you have paid them for the service (which for most private individuals is "nothing"). So if they delete your content, accidentally or deliberately, you've no comeback. I'd put the risk of data loss with Drive as low, but it can never be zero.
If you mean "is it safe from someone else gaining access?", well you are giving the company who operate the storage access (by definition, no matter what their terms say). And as people have said, if the authorities want access you would be foolish in the extreme to assume that the cloud company won't give it to them. So weigh the first one against your risk your personal device falling into someone else's hands - if you are concerned about the second then I'm sure it's against forum rules to give you any advice anyway (we cannot advise people on how to break or circumvent laws here).
As for other privacy risks it's fine as long as your account remains secure (against individual hack or Google suffering a data breach).
In terms of what you can store, you can read their
terms of service. The
program policies link there includes a section on "sexually explicit material" - note that I do not know whether they consider uploading alone to be "publishing". Actually I suspect that if they decided to apply their rules strictly you would not comply with them, since the terms of service require that you are able to give them specific rights to any material you upload, which I'm sure in this case you would not be in a position to grant. In short, if they want to decide that the material violates their rules I'm sure they could find a way. There's also the ambiguity about what they understand by material being "legal", since that has different definitions in different territories.
But there's an old adage that I first met in the 1980s, and is if anything far more relevant now: if you want to keep something private, never put it on a computer that's connected to the internet