Yes they are but it all depends on how you set the security up in the first place. For example, simple Passowrd security can go back right to Acrobat 3, but the level of Encryption decreases and uses only 40 bit RC4 compared to Acrobat X top end 256 bit AES. Also with the later version you have more control on what you want to encrypt for example file attachments.
If you want to use Acrobat certiificate security then this goes back to Acrobat 6.
I would way up your audience and choose the best security settings for your needs, for example there is no point using Acrobats top security method if all of your readers have adobe Reader 8 and cant open the file.
Hope this helps?