It's more secure than centralized systems that can be compromised by hackers, governments or by not following basic security measures like properly hashing passwords. With #nostr you avoid all that to the extent its possible and a private key is way better than what most people would sett as their password. 

 I mean, you cryptographically sign your notes, you encrypt your DMs, so in that regard yes, as long as you keep your nsec private. 

 OK! 

 I say it's secure enough. Though if you are going to be using DMs, know that people know who and when; but not what.